Patterned asymmetric chemical strengthening

ABSTRACT

A glass sheet having asymmetric chemical strengthening is disclosed and described. The examples described herein are directed to a cover glass for an electronic device and other glass components that may be used as enclosure elements or may form an enclosure. Within the glass component, localized compressive stress regions may be formed on opposite sides of the glass component, which may help arrest or redirect propagating cracks or defects in the glass. The opposing compressive stress regions may also help maintain the overall flatness of the component while increasing strength and/or impact resistance of the component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a nonprovisional patent application of, and claimsthe benefit to, U.S. Provisional Patent Application No. 62/697,933,filed Jul. 13, 2018 and titled “Patterned Asymmetric ChemicalStrengthening,” the disclosure of which is hereby incorporated herein inits entirety.

FIELD

The described embodiments relate generally to asymmetric chemicalstrengthening of a glass article. More particularly, the presentembodiments relate to patterned asymmetric chemical strengthening havingan increased depth of compression over at least one localized region.

BACKGROUND

The cover window and display for small form factor devices are typicallymade of glass. Glass, although transparent and scratch resistant, isbrittle and prone to impact failure. Providing a reasonable level ofstrength in these glass parts is crucial to reducing the likelihood ofglass part failure, and hence device failure.

Chemical strengthening has been used to increase the strength of glassparts. Typical chemical strengthening relies on a uniform and symmetricincrease of the compression stress over the entire surface of the glasspart. Such strengthening processes have proven effective at reducingsome level of failure in glass parts. More recently, asymmetric chemicalstrengthening has been established as a method for increasing the depthof compressive stress at local problematic areas of a glass part. Theincreased depth of compressive stress in a glass part affords that areabetter protection against impact related failure. However, asymmetricchemical strengthening, among other things, may lead to warpage in theglass part due to the localized higher compression, which can beexacerbated when the glass part is of a thickness and composition foruse in small form factor devices.

As such, while conventional symmetric and asymmetric chemicalstrengthening are effective, there is a continuing need to provideimproved and alternative ways to strengthen glass, particularly thinglass.

SUMMARY

Various embodiments described herein encompass asymmetricallystrengthened glass articles. Asymmetrically strengthened glass articlescan have enhanced reliability and safety as compared to symmetricallystrengthened glass articles. In embodiments, an asymmetricallystrengthened glass article has a first region with a first stressdistribution, and a second region with a second stress distribution. Thefirst stress distribution and the second stress distribution differ fromone another. For example, the first region may be a first compressivestress region and the second region may be a second compressive stressregion. The differences in the first stress distribution and the secondstress distribution can result in an overall stress imbalance in theasymmetrically strengthened glass article. The overall stress imbalancemay cause the glass article to exhibit warpage. Embodiments hereinrelate to glass articles like cover glass, electronic devices, andmethods that are useful in limiting warpage.

In aspects, a cover glass for an electronic device is described. Thecover glass has a front surface. A first compressive stress regionextends from the front surface to a first depth into the cover glass. Asecond compressive stress region extends from the front surface to asecond depth, less than the first depth, into the cover glass. The coverglass also has a rear surface, which may be opposite to the frontsurface. A third compressive stress region extends from the rear surfacetoward the first compressive stress region and to a third depth into thecover glass. A fourth compressive stress region extends from the rearsurface toward the second compressive stress region and to a fourthdepth, greater than the third depth, into the cover glass.

In embodiments, the cover glass further includes a first tensile stressregion positioned between the first compressive stress region and thethird compressive stress region, and a second tensile stress regionpositioned between the second compressive stress region and the fourthcompressive stress region. In addition, the cover glass can also includea first centerline of the first tensile stress region that is offsetwith respect to a second centerline of the second tensile stress region.

In additional embodiments, the second compressive stress region at leastpartially surrounds the first compressive stress region. The fourthcompressive stress region can at least partially surround the thirdcompressive stress region as well.

In further embodiments, the first depth is approximately equal to thefourth depth and the second depth is approximately equal to the thirddepth. The cover glass can define four corner regions, such that thefirst compressive stress region and the third compressive stress regionare located at least partially within one of the four corner regions. Inaddition, the cover glass can define a rectangular outer perimeterregion, where the first compressive stress region and the thirdcompressive stress region are located at least partially within theouter perimeter region, and the first compressive stress region at leastpartially surrounds the second compressive stress region.

Additional aspects described herein include an electronic devicecomprising a display and an enclosure at least partially surrounding thedisplay. The enclosure may comprise a first localized compressive stressregion extending into the enclosure from a front surface of theenclosure to a first depth, a second localized compressive stress regionadjacent to the first localized compressive stress region and extendinginto the enclosure from the front surface to a second depth, less thanthe first depth, and a rear localized compressive stress regionextending into the enclosure from a rear surface of the enclosuretowards the second localized compressive stress region. The rearlocalized compressive stress region may extend a third depth into thecover sheet that is greater than the second depth. Further, the rearlocalized compressive stress region may be offset with respect to thefirst localized compressive stress region.

In additional aspects of the electronic device, the first localizedcompressive stress region is at least partially surrounded by the secondcompressive stress region. In embodiments, the first localizedcompressive stress region includes potassium ions that extend into thecover sheet a first depth and the second localized compressive stressregion includes potassium ions that extend into the cover sheet at asecond depth that is less than the first depth. Further, the first depthcan be at least twice the second depth.

In embodiments, the enclosure comprises a glass material. The enclosuremay comprise a cover sheet positioned over the display; such as a glasscover sheet. The first and the second localized compressive stressregions may extend from a front surface of the cover sheet and the rearlocalized compressive stress region may extend from a rear surface ofthe cover sheet.

In still other aspects of the electronic device, the cover sheet definesa camera window, and the electronic device has a camera positioned belowthe camera window. The first localized compressive stress region ispositioned at least partially within the camera window and extends intothe cover sheet a first depth, and a second localized compressive stressregion surrounds the first localized compressive stress region andextends into the cover sheet a second depth that is less than the firstdepth.

In some aspects of the electronic device, the cover sheet has a lengthof at least 100 mm and a width of at least 40 mm. The front surface ofthe cover sheet has a flatness that is no more than 120 μm out of plane.

Embodiments herein also include methods of forming a cover sheet for anelectronic device. The method includes positioning a first mask along afirst surface that defines at least a portion of an external surface ofthe electronic device and forming a first compressive stress regionhaving a first thickness along the first surface by exchanging ions intothe cover sheet. The method further includes removing the first mask andforming a second compressive stress region having a second thickness,less than the first thickness, adjacent to the first compressive stressregion by exchanging ions into the cover sheet. The method furtherincludes positioning a second mask along a second surface that isopposite the first surface and forming a third compressive stress regionhaving a third thickness by exchanging ions into the cover sheet. Thethird compressive stress region extends from the second surface towardthe second compressive stress region. The method further comprisesremoving the second mask and forming a fourth compressive stress regionhaving a fourth thickness, less than the third thickness, by exchangingions into the cover sheet. The fourth compressive stress region extendsfrom the second surface toward the first compressive stress region.

In embodiments, an operation of forming a compressive stress regioncomprises immersing the cover sheet in a bath comprising the ions. Thefirst compressive stress region may be formed using a first bath, thesecond compressive stress region may be formed using a second bath, thethird compressive stress region may be formed using a third bath, andthe fourth compressive stress region may be formed using a fourth bath.In some embodiments, the baths all comprise the same type of ions. Inadditional embodiments, the ion composition is substantially the samefor some of the baths, such as first and the third baths and/or thesecond and the fourth baths.

In additional aspects of the method, the operation of forming acompressive stress region comprises immersing the cover sheet in asequence of baths comprising the ions. The baths in the sequence maydiffer in composition. As an example, the cover sheet can comprisealumina silicate glass, and the operation of forming the firstcompressive stress region can comprise immersing the cover sheet into afirst bath comprising sodium ions and subsequently immersing the coversheet in a second bath comprising potassium ions. The first bath caninclude a sodium concentration of greater than 30% mol and the secondbath can include a potassium concentration of greater than 30% mol.

In other aspects of the method, the cover sheet defines four corners,and the first mask leaves each of the four corners exposed along thefirst surface, and the second mask covers each of the four corners alongthe second surface.

Finally, the method may further comprise forming a first tensile stressregion between the first compressive stress region and the fourthcompressive stress region, and forming a second tensile stress regionbetween the second compressive stress region and the third compressivestress region. The first tensile stress region may be offset withrespect to a centerline of the glass sheet in a first direction and thesecond tensile stress region may be offset with respect to thecenterline in a second direction that is opposite to the firstdirection.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be readily understood by the following detaileddescription in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIGS. 1A and 1B depict an example electronic device in accordance withembodiments herein.

FIG. 2 is a flow diagram of a glass strengthening process in accordancewith embodiments herein.

FIG. 3 shows a glass strengthening system in accordance with embodimentsherein.

FIG. 4A is a cross-sectional diagram of a glass cover which has beensymmetrically chemically treated.

FIG. 4B is a cross-sectional diagram of a glass cover which has beensymmetrically chemically treated, as shown to include a chemicallytreated portion in which potassium ions have been implanted.

FIG. 5A is a diagram of a lattice structure for glass.

FIG. 5B is a diagram of a lattice structure for corresponding densifiedglass.

FIG. 6 is a diagram of a partial cross-sectional view of a glass cover,which shows two zones of densified glass.

FIG. 7A is a diagram of a partial cross-sectional view of a glass cover,which shows a tension/compression stress profile in accordance withembodiments herein.

FIG. 7B is a diagram of a partial cross-sectional view of a glass cover,which shows a reduced tension/compression stress profile in accordancewith embodiments herein.

FIG. 7C is a diagram of a partial cross-sectional view of a glass cover,which shows an asymmetric tension/compression stress profile inaccordance with embodiments herein.

FIG. 7D is a diagram of a partial cross-sectional view of a glass cover,which shows an alternative asymmetric tension/compression stress profileas shown in FIG. 7C, and in accordance with embodiments herein.

FIG. 8 is a flow diagram of asymmetric glass strengthening in accordancewith embodiments herein.

FIG. 9 is a cross-sectional diagram of a glass cover which has beenasymmetrically chemically treated.

FIG. 10 depicts an example process for producing an asymmetricallystrengthened glass article using a masking technique.

FIG. 11A depicts a top surface of a cover glass having a silicon nitridecoating applied to the center portion, while the edge and cornerportions remain uncoated.

FIG. 11B is a cross-sectional diagram of a glass cover having acombination of coatings applied to the top and bottom surfaces, thecross-section shown through the cover glass of FIG. 10.

FIG. 12 is a cross-sectional diagram of a glass cover that illustratespatterned asymmetric chemical strengthening.

FIG. 13 is a cross-sectional diagram of a glass cover that illustrateswarpage at an edge of a cover glass due to asymmetric chemicalstrengthening.

FIGS. 14A, 14B, and 14C depict examples of masks applied to the frontand rear surface of a cover glass.

FIG. 15A is a cross-sectional diagram of a cover glass havingillustrative patterned asymmetric strengthening in accordance withembodiments herein.

FIG. 15B is a cross-sectional diagram of a cover glass furtherillustrating patterned asymmetric strengthening in accordance withembodiments herein.

FIG. 15C is a cross-sectional diagram of a cover glass showing patternedasymmetric strengthening and resultant tensile stress in accordance withembodiments herein.

FIG. 15D is a cross-sectional diagram of a cover glass showing patternedasymmetric strengthening around a camera underneath a cover glass inaccordance with embodiments herein.

FIG. 16A is a top surface of a cover glass showing an illustrativepattern for asymmetrically strengthening to avoid cover glass warpage.

FIG. 16B is a bottom surface of the cover glass in FIG. 16A showingpatterned asymmetric strengthening to avoid cover glass warpage.

FIG. 17A is a top surface of a cover glass showing crack propagation toavoid a camera window in accordance with embodiments herein.

FIG. 17B is another illustrative view of a cover glass surfaceasymmetrically strengthened with a pattern to limit crack propagationinto the center of the cover glass.

FIG. 18A is a user or top surface view of a cover glass having a patternto provide chemical strengthening to the corners of the cover glass.

FIG. 18B is an internal or bottom surface view of a cover glass havingan opposing strengthening pattern to FIG. 18A that limits cover glasswarpage and limits crack propagation.

FIG. 18C is a user or top surface view of another cover glass having apattern to provide chemical strengthening to the corners of the coverglass.

FIG. 18D is an internal or bottom surface view of a cover glass havingan opposing strengthening pattern to FIG. 18C that limits cover glasswarpage and limits crack propagation.

FIG. 18E is a user or top surface view of yet another cover glass havinga pattern to provide chemical strengthening to the corners of the coverglass.

FIG. 18F is an internal or bottom surface view of a cover glass havingan opposing strengthening pattern to FIG. 18E that limits cover glasswarpage and limits crack propagation.

FIG. 19 is a block diagram of example components of an exampleelectronic device.

The use of cross-hatching or shading in the accompanying figures isgenerally provided to clarify the boundaries between adjacent elementsand also to facilitate legibility of the figures. Accordingly, neitherthe presence nor the absence of cross-hatching or shading conveys orindicates any preference or requirement for particular materials,material properties, element proportions, element dimensions,commonalities of similarly illustrated elements, or any othercharacteristic, attribute, or property for any element illustrated inthe accompanying figures.

Additionally, it should be understood that the proportions anddimensions (either relative or absolute) of the various features andelements (and collections and groupings thereof) and the boundaries,separations, and positional relationships presented there between, areprovided in the accompanying figures merely to facilitate anunderstanding of the various embodiments described herein and,accordingly, may not necessarily be presented or illustrated to scale,and are not intended to indicate any preference or requirement for anillustrated embodiment to the exclusion of embodiments described withreference thereto.

DETAILED DESCRIPTION

Reference will now be made in detail to representative embodimentsillustrated in the accompanying drawings. It should be understood thatthe following descriptions are not intended to limit the embodiments toone preferred embodiment. To the contrary, they are intended to coveralternatives, modifications, and equivalents as can be included withinthe spirit and scope of the described embodiments as defined by theappended claims.

The following disclosure relates to glass articles or glass components(e.g., cover glass), methods of producing glass articles or glasscomponents, and to the utility of such glass articles in an electronicdevice. Embodiments also relate to the inclusion of asymmetriccompressive stress regions within a glass article in such a way as tomaintain the glass article's flat surfaces while also providing thecapability to direct cracks away from regions of interest or priority inan electronic device, e.g., sensors, cameras, center of the glassviewing zones, etc. In some cases, a front or external surface of theglass article is no more than 120 μm out of plane. In some embodiments,the electronic device can include an enclosure, a display positioned atleast partially within the enclosure, and a glass article, for example acover glass, in accordance with embodiments herein.

In some examples described herein, the glass component or glass articleis a sheet of cover glass for an electronic device. The cover glass maydefine an external and/or internal surface of an electronic device. Theglass article may correspond to a cover glass that helps form part of adisplay area and, in some instances, form part of the enclosure for theelectronic device. In some instances, the glass article or multipleglass articles form the entire enclosure for the electronic device. Theembodiments described herein are particularly relevant for use inportable electronic devices and small form factor electronic devices,e.g., laptops, mobile phones, media players, remote control units, andthe like. Typical glass articles herein are thin, and typically lessthan 5 mm in thickness. In embodiments, the glass articles have athickness from 0.3 to 3 mm, from 0.3 to 2.5 mm, or from 0.1 mm to lessthan 1 mm. However, the dimensions in any particular application mayexceed these example ranges.

As used herein, “glass material” may generally refer broadly to avariety of transparent materials, including substantiallynon-crystalline amorphous solids and/or materials having at least somecrystalline structures, such as glass ceramics of various compositions.Sample compositions of the glass material may include soda lime,aluminosilicate, boro-silicate (and variations thereof), high silicacontent (96% or greater), zinc titanium, or the like. The glass materialmay include other constituent components or may be formed from acomposite material. Typically, the cover glass or other enclosurecomponent includes an ion-exchangeable material, such as soda lime glassor an alkali aluminosilicate glass or glass ceramic.

Reference will now be made to the accompanying drawings, which assist inillustrating various features of the present disclosure. The followingdescription is presented for purposes of illustration and description.Furthermore, the description is not intended to limit the inventiveaspects to the forms disclosed herein. Consequently, variations andmodifications commensurate with the following teachings, and skill andknowledge of the relevant art, are within the scope of the presentinventive aspects.

FIGS. 1A and 1B are perspective diagrams of an electronic device 103.The electronic device 103 may define a top surface 104, bottom surface106, and side surfaces 108. In embodiments, the electronic device 103has a cover, such as a cover glass including a thin sheet of glass witha length and width consistent with the application. As shown in FIG. 1A,the electronic device 103 can have a front cover glass 100 a defining afront surface 102 a. The electronic device may also include a rear coverglass 100 b defining a rear surface 102 b.

In embodiments, the cover comprises a single sheet of glass. In furtherembodiments, the cover may be formed from multiple layers that includeglass sheets, polymer sheets, combinations of glass and polymer sheets,and/or various coatings and layers. In embodiments, the cover may beflexible or bendable.

For purposes of illustration, the electronic device 103 is depicted ashaving an enclosure component 118, a front cover glass 100 a, and a rearcover glass 100 b that together define the device enclosure or housing.In one example, the enclosure component 118 comprises a series of metalsegments that are separated by polymer or dielectric segments thatprovide electrical isolation between adjacent metal segments. It shouldbe noted that the electronic device 103 may also include various othercomponents 120, including, without limitation, speakers, buttons,microphones, one or more ports (e.g., charging ports, data transferports, or the like), touch sensors, cameras, and so on as described infurther detail with respect to FIG. 19.

In an embodiment, the enclosure component 118, the front cover glass 100a, the rear cover glass 100 b, and/or other component of the electronicdevice 103 may be formed from, or include, a cover sheet or otherwise betransparent or have a transparent window region or portion. As shown inFIG. 1A, the front cover glass 100 a defines the entire front face orsurface of the electronic device 103.

As shown in FIGS. 1A and 1B, the enclosure component 118, the frontcover glass 100 a, and the rear cover glass 100 b are three separate anddistinct components that together define an enclosure of the electronicdevice 103. However, in some embodiments, the enclosure component 118,the front cover glass 100 a, and the rear cover glass 100 b are formedtogether as a single monolithic structure or component. For example, thesingle monolithic glass component may define a portion of a sidewall ofthe enclosure and optionally a front and/or rear surface of theenclosure. In addition, a single monolithic glass component may form thefront, rear, top, bottom, and/or side surfaces of the enclosure of theelectronic device 103. In another alternative embodiment, the enclosurecomponent 118 defines the entire rear face or surface of the enclosure,as well as the top, bottom, and/or the sides of the enclosure.

One or both of the front and rear cover glass 100 a, 100 b may define atransparent window region. A transparent window region may extend over adisplay component, a camera, an optical sensor, or another optical orvisual device 120. For example, a front cover glass 100 a may bepositioned over a display component that is configured to produce agraphical output that is viewable through a transparent window region ofthe cover member. In some instances, a touch-sensitive layer (e.g., acapacitive touch sensor) is attached to the cover glass and positionedbetween the cover glass and the display component. Further, one or bothof the front and rear cover glass may include one or more openings for acamera, light source, or other optical component.

In general, a transparent window region may be a portion of the coverglass 100 a, 100 b that is free from markings, textures, inks, and soon. In some cases, the transparent window region may be a transparentportion of a cover glass that may have substantially opaque regionsadjacent the transparent window region. It will be appreciated thatother non-window portions, including substantially all of the coverglass 100 a, 100 b, may also be free from markings, textures, inks, andso on, as may be appropriate for a given application. In some cases,other portions 112 of the cover glass 100 a, 100 b may be partiallycovered by an ink or marking and, in some cases, may be translucent,opaque, or otherwise not perfectly transparent.

Each piece of cover glass 100 a, 100 b can have front and rear surfaces,respectively, and can be composed of regions, zones, and/or portions.For example, one region of a front cover glass 100 a could correspond tothe entire front surface 102 a. Another region of the front cover glass100 a could be an area corresponding to one or more edges 110 of theglass. In some cases, this is referred to as a peripheral region or arectangular peripheral region for a rectangular front cover glass 100 a.A region or zone having the same glass attributes can be continuous; forexample, all four edges of the cover glass may be representative of asingle region or zone. A region or zone having the same glass attributescan also be discontinuous, for example, the four corners 114 of thefront cover glass 100 a. The strength requirements for the surfaces andregions may differ on the use; for example, a front surface 102 a,exposed to the outside environment, may require a different strengththan the rear surface, enclosed away from the environment.

The differential strength requirements of a cover glass can be addressedusing patterned asymmetric chemical strengthening, as described infurther detail below, which can also be used to maintain a certain levelof glass article flatness. With respect to flatness, in embodiments, aglass surface is flat if the glass surface is no more than 120 μm out ofplane. In additional embodiments, a portion or region of the glasssurface is flat to within a specified extent. For example, when thecover glass includes a bend or curve (e.g., the cover glass of FIG. 13),a central region of the cover glass may be flat as specified. Thisspecification may be applied to devices that have a cover glass with awidth that is at least 40 mm and a length that is at least 100 mm. Insome cases, patterned asymmetric chemical strengthening can also be usedto direct impact produced crack propagation away from the impact site toother regions in the glass of lower glass article priority.

Embodiments herein are discussed below with reference to FIGS. 2-18F.However, those skilled in the art will readily appreciate that thedetailed description given herein with respect to these Figures is forexplanatory purposes only and should not be construed as limiting.

Chemical Strengthening

Embodiments herein may utilize a glass chemical strengthening processwhere a glass article is first enhanced by immersion in a first ionsolution (sodium, for example) and then strengthened by immersion in asecond ion solution (potassium, for example). These processes can bothbe used to strengthen a glass article, as well as to direct or controlimpact created crack propagation within the glass article, all whilekeeping the glass article surfaces flat (e.g., limiting surfacewarpage).

FIG. 2 is a flow diagram of a glass strengthening process 200 accordingto some embodiments. In embodiments, glass strengthening process 200 mayuse ion exchange to form a pattern of asymmetric compressive stressregions in a piece of glass. As shown in FIG. 2, glass strengtheningprocess 200 begins with operation 202 of obtaining the piece of glass200.

The glass strengthening process 200 further includes an operation 204 ofenhancing the glass. The glass may be enhanced through chemicalprocessing. For example, operation 204 may comprise a first ion-exchangeoperation. The first ion-exchange operation may use a first ion-exchangebath.

The glass strengthening process 200 further includes an operation 206 ofchemically strengthening the glass through further chemical processing.For example, operation 206 may comprise a second ion-exchange operation.The second ion-exchange operation may use a second ion-exchange bathdifferent from the first ion-exchange bath.

In embodiments, the operations of glass strengthening process 200 mayinclude additional features of the present disclosure, such as featuresdescribed with respect to FIGS. 3 and 10. Further, glass strengtheningprocess 200 may include additional operations, such as maskingoperations.

FIG. 3 illustrates one embodiment for strengthening a glass article 300in accordance with embodiments herein. A glass article 302 in need ofglass strengthening is immersed in a first bath 304 that contains asodium solution 306 comprising sodium ions. The enhanced strengthenedglass article is then removed from the first bath 304 and immersed in asecond bath 308 that contains a potassium solution 310 comprisingpotassium ions. In some embodiments, the strengthened glass article canbe quenched to eliminate further exchange of ions from the treated glassarticle. In some cases, one or more surfaces of the glass article 302are masked and/or have been treated to enhance or suppress ion exchangealong a localized region. A glass article treated using this method ofstrengthening would have little or no warpage and have little or nocontrol over the direction of impact initiated crack propagation.

The level of glass article enhancement is generally controlled by thetype of glass (glass articles can, for example, be alumina silicateglass or soda lime glass, and the like); the sodium ion or sodium saltconcentration of the bath (e.g., sodium nitrate, typically 30%-100%mol); the time the glass article spends in the bath (typically 4-8hours); and temperature of the bath (350° C.-450° C.).

Strengthening of the glass article in the second bath is controlled bythe type of glass, the potassium ion concentration, the time the glassspends in the solution, and the temperature of the solution. Here, thepotassium ion or potassium salt concentration (e.g., potassium nitrate)is in the range of 30-100% mol, but the glass article would remain inthe bath for about 6-20 hours at a bath temperature of between about300° C.-500° C.

Generally, chemical strengthening processes rely upon ion exchange. Ineach solution or bath, the ions therein are heated to facilitate ionexchange with the glass article 302. During a typical ion exchange, adiffusion exchange occurs between the glass article 302 and the ion bath304, 308. For example, sodium ions in the sodium solution 306 of thefirst bath 304 may provide an exchange enhancement process. Inparticular, the sodium ions may diffuse into the surface of the exposedglass, allowing a build-up of sodium ions in the surface of the glass.In embodiments, the sodium ions replace other ions found in a silicate(e.g., aluminosilicate) or soda lime glass. In embodiments, sodium ionsmay exchange for smaller lithium ions in the glass. The ion exchangeduring immersion in the first bath may take place at a first temperaturebelow a glass transition temperature of the glass.

Upon immersion of the enhanced glass article 302 into the potassiumsolution 310 of the second bath 308, the sodium ions of the enhancedglass article 302 are replaced by potassium ions in surface areas to agreater extent than sodium ions found more toward the interior or middleof the glass article 302. The ion exchange during immersion in thesecond bath may take place at a second temperature below the glasstransition temperature of the glass. After exchange of sodium ions inthe glass for potassium ions, a compression layer is formed near thesurface of the glass article 302 (for example, the larger potassium ionstake up more space than the exchanged smaller sodium ions). The sodiumions that have been displaced from the surface of the glass article 302become part of the potassium bath ion solution.

Depending on the factors already discussed above, a compression layer asdeep as about 10-100 microns (μm), and more typically 10-75 μm, can beformed in the glass article 302. In some embodiments, a deepercompression layer may be formed, such as from 100 microns to 250microns.

In general, the preparation of a compression layer may result inincreased volume in targeted zones of the glass article 302, which canresult in warpage of the glass article 302. Where the compression layeris prepared to a uniform or consistent depth over both surfaces of theglass article 302, warpage is of limited concern, as the ions will exertthe same force over the entire surfaces of the glass article 302. Whereasymmetric chemical strengthening is utilized, as discussed below, theions exert a non-uniform force over the surfaces of the glass article302, which can result in warpage or bending of various areas of theglass article 302. However, by using patterns of asymmetricallystrengthened compression layers to strengthen different zones or regionsof the glass article 302, flat surfaces can be maintained, and controlof any impact damage away from priority areas of the glass can beaccomplished. In general, patterns of compressive stress regions can beinput into the glass article 302 to accomplish the strengthening aspectsin the glass, while also used to oppose each other and limit warpage,and provide barriers to redirect, reduce, or prevent crack propagation.

FIG. 4A is a cross-sectional diagram of a glass article 400 which hasbeen chemically treated such that a symmetrically chemicallystrengthened layer 402 is created. The glass article 400 includes achemically strengthened layer 402 and a non-chemically strengthenedinner portion 404. While discussed in greater detail throughout, theeffect of chemically strengthening the glass article 400 as shown inFIG. 4A is that the inner portion 404 is under tension, while thechemically strengthened layer 402 is in compression. The chemicallystrengthened layer 402 has a thickness (Y) which may vary depending uponthe requirements of a particular use. Note that the forces of thechemically strengthened layer 402 are uniform on the glass article 400such that little or no warpage would occur.

While the simplified representation of the chemically strengthened layer402 is depicted as having a uniform thickness, in accordance withembodiments described herein, the chemically strengthened layer 402 maybe formed from a series of compressive stress regions, at least some ofthe compressive stress regions having a different thickness (or depth oflayer). As described in more detail herein, the series of compressivestress regions may be distributed or positioned along the front and rearsurfaces of the glass article 400 to help reduce or prevent warpage toproduce a substantially flat glass article 400.

FIG. 4B is a diagrammatic representation of a chemically strengthenedprocess. Note that some amount of sodium 405 diffuses from the enhancedglass article to the ion bath, while potassium (K) ions 406 diffuse intothe surface of the glass article, forming the chemically strengthenedlayer 402. Alkali metal ions like potassium, however, are generally toolarge to diffuse into the center portion of the glass article, therebyleaving the inner portion 404 only under tension and not in compression.By controlling the duration of the treatments, temperature of thetreatments, and the concentration of the various ions involved in thetreatments, the thickness (Y) of a chemically strengthened layer 402 maybe controlled, as well as the concentration of ions in the chemicallystrengthened layer 402. Note that the concentration of the ions involvedin the chemical strengthening process may be controlled by maintaining,during glass article treatment, a substantially constant amount of ionsin each of the two baths (for example, as the potassium ions diffuseinto the glass, a controller would add more potassium ions into the ionbath—thereby encouraging the potassium to continue to diffuse into theglass). The relationship between the chemically strengthened compressionlevel (both ion concentration at the surface and depth) and the innertension portion forms a stress pattern for a chemically treated glassarticle.

Additional ion bath immersions may be added to the basic glass chemicalstrengthening process. For example, a third bath including sodium ions(e.g. from sodium nitrate) can be used to immerse the strengthened glassso as to exchange potassium ions out of the compression layer for sodiumions in the third bath. This is referred to as a back-exchange ortoughening process. The toughening process is used to further controlthe depth and strength of a compression layer, and, in particular, toremove some compression stresses from near the top surface regions,while allowing the underlying potassium ions to remain in the lowerregions of the compression layer. In addition, the toughening processreduces the central tension from the glass article (see below).

Although sodium enhancement and potassium strengthening are describedherein, other ion combinations are within the scope of the presentdisclosure, for example, use of lithium instead of sodium, or cesiuminstead of potassium, e.g., sodium-potassium, sodium-cesium,lithium-potassium, lithium-cesium treatment combinations. Any ioncombination can be used herein that provides an increase in the glassarticle surface compression and compression depth.

Chemical strengthening is applied to glass surfaces, and relies uponexposure of the glass surface to the chemical strengthening process.Where a glass article is immersed such that all aspects of the articlehave equal exposure to the ion bath, the glass article surface will besymmetrically strengthened, allowing for a glass article with auniformly thick and composed compression layer (Y) and little or nowarpage.

In accordance with some embodiments described herein, a glass articlesurface may not be equally exposed to chemical strengthening resultingin a surface that is asymmetrically strengthened. More specifically, thetechniques described herein allow for a glass article with a non-uniformcompression layer while still maintaining flatness or reducing thepotential for part warpage. As above, asymmetrically strengthened glassarticles have a stress pattern; however, the stress pattern is modifiedbased on the asymmetry of the chemical treatment and, as described ingreater detail below, patterned asymmetric chemical strengthening isused to avoid glass article warpage, and thereby maintains substantiallyflat surfaces. Patterns of asymmetric chemical strengthening aretherefore used to strengthen a zone of a glass article, avoid glassarticle warpage, and provide an impact pathway that avoids priorityaspects or regions of the glass article.

Tools for Asymmetric Chemical Strengthening: Pre-Heating to IncreaseGlass Density Prior to Chemical Strengthening

Chemical strengthening may be enhanced or facilitated by various thermaltechniques that are performed prior to the chemical strengtheningprocess. Chemical strengthening is limited by the saturation limit ofthe glass for an amount or volume of ions. The size, depth, andconcentration of ions within a glass article directly relate to thecharacteristic strengthening for the glass which, as described herein,can be modified and calibrated throughout the glass to prepare the glassfor a particular use.

At saturation, no additional compression layer or depth modificationsmay be accomplished (via diffusion). However, modification of thermalinput to a glass article, prior to chemical strengthening, can allow forenhancement of the glass surface density, which will directly contributeto the concentration and depth of the strengthened compression layer.

Where a significant amount of thermal energy is added to a glass articleprior to chemical strengthening, the glass density of the article can beincreased. Glass density in these embodiments results in the glasslattice being heated to a point of densification. With regard to theembodiments described herein, localized densification can be used toproduce asymmetric chemical strengthening. In particular, localizeddensification can increase or decrease susceptibility or sensitivity ofa particular region to an ion-exchange process and, therefore, be usedto create localized compressive stress regions having a distinctthickness or other characteristic.

As schematically shown in FIG. 5A and FIG. 5B, denser glass (FIG. 5B)500 provides a more limited lattice structure (more restricted and lessflexible) and is less able to undergo ion diffusion to deeper levelsthan non-treated glass (FIG. 5A) 502. In FIGS. 5A and 5B, the glass hasa starting glass lattice structure 502, which when heated to adensification temperature, is densified and provides a smaller volume506 for ions to move through than the volume 508 of the non-densifiedglass 502. In an embodiment, the lattice structure is a networkstructure, such as a silicate-based network structure. For example, analuminosilicate glass may have an aluminosilicate network structure. Therestriction on the glass lattice allows for fewer ions to diffuseinwardly, while the concentration of ions in the chemical strengtheningbath remains high (as compared to an ion bath used for non-densifiedglass). Also, although the glass lattice has been densified, embodimentsherein do not result in thermal input to the point of crystal latticecollapse (not shown), rather as heat is applied to the point of latticelimitation, some ions are able to diffuse into the glass. The ions thatdo diffuse into the glass are tightly packed at the surface of thedensified glass and thereby provide a superior surface compression layerof shallow depth.

As such, the increase in glass density at the start of the chemicalstrengthening process limits ion diffusion into the glass surface,allowing the glass to exchange a greater amount of ions at the surfaceof the glass, but only allowing the exchange to a shallow depth. Glassarticles treated prior to chemical strengthening by initial thermalinput typically express a higher chemical stress at the surface, but toa shallower depth. These glass articles are most useful for highcompressive stress but to a shallow depth, e.g., an article wherepolishing or other like procedure is likely required on the chemicallystrengthened glass, or where the glass may be exposed to increased riskof scratching but not wear and tear (impact).

One such thermal technique is annealing a glass article prior tochemical strengthening. Annealing includes subjecting the glass articleto a relatively high temperature in an annealing environment for apredetermined amount of time, and then subjecting the glass article to acontrolled cooling for a second predetermined amount of time. Onceannealed and chemically strengthened, the glass article will have amodified compressive stress as compared to similar glass articles notannealed prior to chemical strengthening. As noted above, annealing isparticularly important where the glass article is in need of highsurface compressive stress (but to a shallower depth).

The annealing process requires that the glass article be heated to atemperature between the strain point temperature and softeningtemperature of glass, also known as the annealing temperature (foraluminosilicate glass, the annealing temperature is between about 540°C.-550° C.). The time required to anneal a glass article varies, but istypically between 1-4 hours, and cooling times typically are on theorder of ½° C./min for up to about 5 hours.

Typically, glass articles that have been annealed may be taken straightfrom a controlled cooling and immersed in the enhancement ion bath(sodium), or, alternatively, the article may be further air cooled, andthen immersed in the first ion bath. Once annealed, the glass willresist deeper ion diffusion but allow some diffusion at the surface. Thediffusion into the surface allows for high compression stress (withshallow depth).

A second thermal technique used to raise a glass article's density priorto chemical strengthening is hot isostatic pressing or HIP. HIP includessimultaneously subjecting the glass article to heat and pressure for apredetermined amount of time in an inert gas. The glass article isallowed to remain in the HIP pressure vessel until the glass article isdenser, where internal voids in the glass are limited. As for annealing,the increase in glass density prior to chemical strengthening by HIPallows for the production of a higher compression stress at the glassarticle surface, but to a shallower depth (than would be expected for aglass article that does not undergo HIP).

HIP parameters vary, but an illustrative process would involve placingthe glass article to be chemically strengthened in a HIP pressurevessel, drawing a vacuum on the vessel, and applying heat to the glassarticle in the vessel. Under pressure, the vessel may be heated to600-1,450° C., depending on the type and thickness of the glass. Heatand pressure are typically maintained for about 10-20 minutes, afterwhich the processed glass is allowed to cool. In some embodiments, asuitable inert gas can be introduced in the vessel to facilitate heatingof the glass article. HIP is another tool for modifying or enhancing thechemical strengthening process.

As shown in FIG. 6, the pre-heating of the glass article 600 can belocalized (and not across the entire surface(s) of the glass article),such that target or predetermined regions 602 of the glass article aredensified. In this embodiment, localized heating (shown as arrows 604)is performed prior to chemical strengthening and to a point between thestrain point temperature and softening temperature of the glass. Laseror inductive coil heating can be used to pre-heat the location andthereby provide a glass article that includes both densified 608 andnon-densified glass surfaces 610. FIG. 6 shows a simple cross-section ofa glass cover 600 where the sides have been locally pre-heated to formdensified glass 608, while the center of the glass article exhibitsnon-densified glass 610.

Embodiments herein include glass articles pre-treated by heatingtechniques to form densified glass over an entire surface, or inpredetermined regions or locales, leaving regions of different glassdensity. When a glass article so treated is chemically strengthened 612,the article will be asymmetrically strengthened and have an asymmetricstress pattern, where densified glass exhibits a higher surfacecompression stress, but to a shallower depth, than correspondingnon-densified glass. It is envisioned that the timing and placement ofthe pre-heating can be used to optimize a glass surface compressivestress and the depth of the compressive stress.

Although not explicitly noted in all embodiments herein, all glassarticle embodiments herein may include the use of glass articles thathave been pre-heated to densify the glass prior to chemicalstrengthening. As noted above also, so treated glass may exhibit warpagedue to the asymmetric strengthening of densified and non-densifiedglass, essentially allowing for production of different regions withdifferent levels and depths of stress.

Stress Profiles

Chemically treating a glass article in accordance with embodimentsherein effectively strengthens the exposed or treated surfaces of theglass. Through such strengthening, glass articles can be made strongerand tougher so that thinner glass can be used in portable electronicdevices.

FIG. 7A is a diagram of a partial cross-sectional view of a glassarticle, for example a cover glass. The diagram shows an initialtension/compression stress profile according to one embodiment. Theinitial tension/compression stress profile may result from an initialexchange process to symmetrically strengthen the surface region of theglass. A minus sigma legend indicates a profile region of tension, whilea plus sigma legend indicates a profile region of compression. Thevertical line (sigma is zero) designates crossover between compressionand tension.

In FIG. 7A, thickness (T) of the cover glass is shown. The compressivesurface stress (CS) of the initial tension/compression stress profile isshown at the surface of the cover glass. The compressive stress for thecover glass has a compressive stress layer depth (DoL) that extends fromsurfaces of the cover glass towards a central region. Initial centraltension (CT) of the initial tension/compression stress profile is at thecentral region of the glass cover. In embodiments, thetension/compression stress profile (or stress profile) extends acrossthe thickness of the glass. In further embodiments, only the compressionportions of the stress profile (or compressive stress distributions) maybe determined.

As shown in FIG. 7A, the initial compressive stress has a profile withpeaks at the surfaces 700 of the cover glass 702. That is, the initialcompressive stress 704 is at its peak at the surface of the glass cover.The initial compressive stress profile shows decreasing compressivestress as the compression stress layer depth extends from surfaces ofthe glass cover towards the central region of the glass cover. Theinitial compressive stress continues to decrease going inwards untilcrossover 706 between compression and tension occurs. In FIG. 7A,regions of the decreasing profile of the initial compressive stress ishighlighted using right-to-left diagonal hatching.

The peaks at the surface of the cover glass provide an indication of thebending stress a cover glass can absorb prior to failure, while thedepth of the compressive stress region provides protection againstimpact. After crossover between compression and tension, a profile ofthe initial tensile stress region 708 extends into the central regionshown in the cross-sectional view of the cover glass. In the diagram,FIG. 7A, regions of the decreasing profile of the tensile stress region(CT) extending into the central region is highlighted using hatching.

Typically the combinations of stresses on a glass article are budgetedto avoid failure and maintain safety. For example, if you put too muchcompressive stress into a glass article, the energy will eventuallycause the article to break or fracture. Therefore, each glass articlehas a stress budget, an amount of compressive versus tensile strengththat provides a safe and reliable glass article. In FIG. 7A, thecompressive stress on the glass article is fairly balanced on the top(front) surface and bottom (rear) surface and equal to the tensilestress. As such, the glass article will avoid or have very limited to nowarpage.

FIG. 7B is a diagram of a partial cross-sectional view of a cover glass,which shows a reduced tension/compression stress profile according toone embodiment. The reduced tension/compression stress profile mayresult from a double exchange process. Reduced compressive surfacestress (CS′) of the reduced tension/compression stress profile is shownin FIG. 7B. The compressive stress layer depth (DoL) now corresponds tothe reduced compressive stress. In addition, reduced central tension isshown in the central region.

In light of FIG. 7B, it should be understood that the reducedcompressive surface stress (CS′) shows increasing profiles as thecompressive surface layer depth extends from surfaces of the cover glassand towards the submerged profile peaks. Such increasing profiles ofcompressive stress may be advantageous in arresting cracks. Within adepth (DoL) of the submerged peaks, as a crack attempts to propagatefrom the surface, deeper into the cover glass, it is met with increasingcompressive stress (up to DP), which may provide crack arresting action.Additionally, extending from the submerged profile peaks further inwardtoward the central region, the reduced compressive stress turns toprovide a decreasing profile until crossover between compression andtension occurs. As in FIG. 7A, the reduced compressive stress issymmetric and avoids warpage at the surfaces of the glass article.

FIGS. 7A and 7B show a symmetric stress profile at a particular locationwithin the glass article, where both sides (front and rear) of the coverglass have equal compressive stress, compressive stress layer depth, andtensile stress. While portions of a glass article may have a symmetricprofile, other regions or areas of the glass article may have anasymmetric profile, which may help to create crack diversion orlocalized strengthened zones within the glass article.

FIG. 7C shows an asymmetric stress profile for a first zone 741 of acover glass 714 where the front surface 716 shows a more significantcompressive stress region 721 having depth (DoL₁) than the compressivestress region 723 having depth (DoL₃) from the rear surface 718. Notethat the front surface 716 would, in this case, be more durable andimpact resistant than the rear surface. Also note that there is a stressbudget; the inclusion of additional compressive stress on the frontsurface may be compensated for by a much shallower depth of compressionon the rear surface.

As shown in FIG. 7C, the increased depth of the compressive stressregion 721 along the front surface 716 results in a tensile stressregion 731 that is offset with respect to the centerline 729 of thecover glass 714. In particular, the tensile stress region 731 is offsetaway from front surface 716 or toward the rear surface 718. Thedistribution of tensile stresses in tensile stress region 731 need notbe uniform, as schematically shown in FIG. 7C. In embodiments, themaximum value of the tensile stress (CT₁) in tensile stress region 731is also offset with respect to the centerline 729 (away from the frontsurface 716 and towards the rear surface 718). In embodiments, acenterline of the tensile stress region may be halfway between thecross-over points between compression and tension.

To facilitate a downward shift of the tensile stress region 731, thecompressive stress region 723 along the rear surface may be thinner orhave a reduced depth as compared to the thickness or depth of thecompressive stress region 721 along the front surface 716. For example,DoL₃ may be from 5 microns to 50 microns or greater than 20 microns to50 microns while DoL₁ may be from 100 microns to 250 microns. Creating athinner compressive stress layer can reduce the integral of the tensilestress over tensile stress region 720, which may enhance the reliabilityof the cover glass 714.

In embodiments, the maximum compressive stress CS₁ of compressive stressregion 721 (along the front surface) is greater than the maximumcompressive stress CS₃ of the compressive stress region 723 (along therear surface). The maximum compressive stress of the compressive stressregions may be located at the surface of the cover glass. As examples,the maximum compressive stress CS₁ may be from 600 MPa to 800 MPa andthe maximum compressive stress CS₃ may be from 300 MPa to less than 600MPa, greater than 400 MPa to less than 600 MPa, or from 450 MPa to lessthan 700 MPa. In further embodiments, CS₁ of compressive stress region721 (along the front surface) is about equal to the maximum compressivestress CS₃ of the compressive stress region 723 (along the rearsurface).

As described in more detail below with respect to FIGS. 15A-15D,asymmetric stress regions may be located adjacent to each other in orderto help balance the internal stress and help maintain part flatness orreduce warpage. In particular, and as shown in FIG. 7D, an adjacent zone742 may include a thinner compressive stress region 722 near the frontsurface 716 and a thicker compressive stress region 724 along the rearsurface 718. This complementary stress pairing, if located adjacent tothe asymmetric profile depicted in FIG. 7C, may help balance theinternal stresses. For example, the asymmetry of compressive stress inzone 742 may at least partially counteract the asymmetry of compressivestress in zone 741 and reduce warping of the cover glass.

In some cases, the adjacent zone 742 will have a tensile stress region732 that is offset from the centerline 729 of the cover glass 714 in adirection opposite to the direction of offset of the tensile stressregion 731 of FIG. 7C. Specifically, the adjacent zone 742 will have atensile stress region 732 that is offset from the centerline 729 in adirection which is upwards or toward the front surface 716. Similarly,the maximum value of the tensile stress (CT₂) in tensile stress region732 is also offset with respect to the centerline 729 (towards the frontsurface 716 and away from the rear surface 718). These offset tensilestress regions may further facilitate the reduction of crack propagationthrough the pair of zones. For example, a crack which begins topropagate from front surface 716 in zone 742 may tend to remain in zone742 if the compressive stress in compressive stress region 722 is lessthan the compressive stress at an equivalent depth in compressive stressregion 721. In addition, if the crack propagates beyond compressivestress region 722, the crack may tend to remain in tensile stress region732 rather than enter compressive stress region 721 or a comparativelylow tension portion of tensile stress region 731. In some embodiments,CT₁ is about equal to CT₂, while in additional embodiments CT₁ maydiffer from CT₂.

The depth of layer DoL₂ of compressive stress region 722 may be from 5microns to 50 microns or greater than 20 microns to 50 microns while thedepth of layer DoL₄ of compressive stress region 724 may be from 100microns to 250 microns. In embodiments, the maximum compressive stressCS₄ of compressive stress region 724 (along the rear surface) is greaterthan the maximum compressive stress CS₂ of the compressive stress region722 (along the front surface). The maximum compressive stress of thecompressive stress regions may be located at the surface of the coverglass. As examples, the maximum compressive stress CS₄ may be from 600MPa to 800 MPa and the maximum compressive stress CS₂ may be from 300MPa to less than 600 MPa, greater than 400 MPa to less than 600 MPa, orfrom 450 MPa to less than 700 MPa. In further embodiments, CS₂ ofcompressive stress region 722 (along the front surface) is about equalto the maximum compressive stress CS₄ of the compressive stress region724 (along the rear surface).

As will be discussed in greater detail below, design and production ofcover glass having modified stress profiles like FIGS. 7C and 7D forcalibrated utility are accomplished by using patterned asymmetricchemical strengthening processes described herein. By asymmetricallystrengthening a cover glass exhibiting target patterns, highly usefulcover glass may be produced. In such instances, the stress pattern forany zone or region of the cover glass may be used to provide a stressprofile, and, therefore, cover glass, having an optimized surface forits utility. By combining stress profiles in specific patterns atdifferent zones or regions of the cover glass, cover glass having theappropriate strength, and lack of warpage for a particular use, can beprepared. In addition, targeted asymmetric strengthening can be used todirect impact failure toward less prioritized areas or regions of thecover glass, for example, by providing a path of patterned asymmetricstrengthening from regions of likely impact, to propagate to an area inthe cover glass having lower priority, i.e., where a crack has a lowercapacity to interfere with the overall utility of the cover glass.

Asymmetric Chemical Strengthening

Embodiments herein result in the production of asymmetricallystrengthened glass articles showing various patterns that have little tono glass warpage and facilitate glass cracking patterns to avoidpriority regions of the glass article. Asymmetrically strengthened glassarticles, for example, cover glass, using patterns described herein, canbe designed to be more reliable, damage resistant, flat, and safer thancorresponding symmetrically strengthened or asymmetrically strengthenedglass articles.

FIG. 8 shows an illustrative flow diagram for asymmetricallystrengthening a glass article 800. In particular, FIG. 8 depicts aprocess that can be used to produce or design an asymmetricallystrengthened glass article. In operation 802, a glass article isidentified for a desired utility based on its dimensions, its thickness,and its inherent composition. In operation 804, a budget for how muchstress the identified glass can withstand is determined based on theutility of the glass, and a budget determined for optimal reliabilityand safety for the glass. For example, the stress budget may includebalancing an amount of stress in the glass to provide both strength andsafety 806. In operation 808, the glass article is then calibrated toexhibit a useful stress pattern so as to maximize the stress budget andutility through use of asymmetric chemical strengthening. As part of thecalibration and in operation 810, the pattern is also designed to resultin the stress being balanced to allow for reduction and/or eliminationof substantially all warpage in the glass article, and to provide one ormore impact pathways away from high priority regions of the glassarticle (e.g., to facilitate crack propagation to regions of the glassarticle having less of an impact on the utility of the glass). Patternsare therefore designed by identifying regions where chemicalstrengthening is required and corresponding zones on the same oropposing surfaces that can be used to oppose the bending forces andthereby result in a net cancelling out of surface bending. Regions canalso be incorporated to provide lower barriers to a propagation crack,such that high impact areas have release regions or pathways to direct acrack away from a particular region (e.g., a transparent window) of theglass.

For example, a piece of thin cover glass positioned over a display of aportable electronic device optimally requires different properties overits two surfaces, front and rear. Asymmetry of the chemicalstrengthening may be required on the front-versus the rear-side of acover glass, on the perimeter versus the center of a cover glass, aroundfeatures in a cover glass, and in hard-to-polish areas in a cover glass.However, as discussed above, each cover glass has a stress pattern toavoid failure, where the compressive stress and tensile stress must beroughly balanced. As such, asymmetric chemical strengthening is used tooptimize the properties of a particular cover glass, within the stressbudget of the cover glass, for a particular use. In addition, thepattern of compressive stress regions to tensile stress regions is alsopatterned to facilitate and substantially reduce or eliminate warpage,and to provide crack propagation pathways to avoid a crack moving into aregion where the cover glass would be considered a failure.

In general, patterns of asymmetric chemical strengthening can be used toprovide a higher (or lower) surface compression region to a deeper (orshallower) depth, for a particular region, while opposing that surfacecompression layer with opposing stress to maintain a cover glass havinglittle or no warpage, and useful crack or impact release pathways awayfrom prioritized zones on the glass article. In embodiments, therelationship of the compressive stress regions (amount and depth) on thefront and rear surfaces of a cover glass in relationship to theresultant tensile stress regions gives a stress pattern for the coverglass. The stress pattern can be along the X, Y, or Z axis of the coverglass. In embodiments, the stress pattern has a lateral component (e.g.,along the X and/or Y axis) as well as a thickness component (e.g., alongthe Z axis). Forces exerted in the cover glass are used to oppose eachother and provide substantially flat surfaces.

For example, a cover glass can have a front surface and a rear surfaceopposite to the front surface. A first compressive stress region extendsinto the cover glass from the front surface to a first depth. A secondcompressive stress region extends into the cover glass from the frontsurface to a second depth, the second depth being less than the firstdepth. A third compressive stress region extends into the cover glassfrom the rear surface toward the first compressive stress region, andthe third compressive stress region has a third depth. A fourthcompressive stress region extends into the cover glass from the rearsurface toward the second compressive stress region to a fourth depth,and the fourth depth is greater than the third depth. The combination ofthe four compressive stress regions, each having an independent depth,can be used to both strengthen the cover glass, as well as to keep thefront and rear surfaces flat.

As such, in embodiments herein, asymmetric chemical strengthening of aglass article is provided to: increase the reliability of a glassarticle for a particular use; increase the safety of a glass article fora particular use; facilitate flat or substantially flat surfaces of aglass article; provide crack propagation pathways for reducing theeffect of an impact; and other like utilities.

FIG. 9 shows that asymmetric chemical strengthening, in general, isdependent on differentially incorporating ions into a surface of a glassarticle, like a cover glass. As noted above, a cover glass 900, alongany surface area 902, can exchange and incorporate ions to a particulardepth and concentration based on the density and overall ion saturationpoint of the cover glass. In general, there is only so much volume inthe glass that can be involved in the exchange to larger-sized ions soas to increase the compression of the glass (see 901 versus 903). Thechange in ion concentration along the surface, and to particular depths,modifies the internal stress relationship of the glass, and thisrelationship extends across the thickness of the glass 904, as well asthroughout the interior portion of the glass (how the internaltension/compression stress changes across the middle of the glassarticle) 906. As such, and as discussed previously, a stress pattern canbe across the thickness of a glass article (vertical—top to bottomsurface) 904 as well as across or throughout the glass article(horizontal—side to side) 906.

Embodiments herein utilize these stress relationships to calibrateutilities to provide modified glass articles for use in portableelectronic devices and small form factor devices. In addition, thesestress relationships can be used to avoid warpage in the glass article,and to provide pathways for crack propagation away from prioritizedregions of the glass article. Surprisingly, the combination of glassstrengthening can accomplish all three priorities: strengthening of theglass, avoidance of warpage in the glass, and capacity to direct impactdamage away from prioritized regions in the glass.

Patterns of Asymmetric Strengthening Via Masking or Coating

Embodiments herein include the application of masking or ion-diffusionbarriers to portions of a glass article prior to immersion in the ioncontaining baths. For example, a portion of the glass surface can bephysically masked from the ions in the chemical strengthening processvia a diffusion of impermeable material, such as a metal or ceramic,sealed over the region where diffusion is not wanted. This type ofphysical masking can partially or completely limit ion-diffusion intothat surface and provides asymmetric strengthening, i.e., the maskedsurface will receive little to no ion exchange as compared to the otherexposed surfaces of the glass article. Once chemically treated, thephysical barrier would typically be removed from the glass article. Hereyou would have treated and untreated surfaces. The application ofmasking and/or ion-diffusion barriers can be used to apply a pattern toa region or regions in the glass article.

In aspects, the methods described herein comprise two ion-exchangeoperations, with the mask(s) being removed from the article between thetwo ion-exchange operations. The two ion-exchange operations may takeplace in two different ion-exchange baths. In additional aspects, themask is removed from the article after multiple ion-exchange operationsare performed.

FIG. 10 depicts an example process 1000 for producing an asymmetricallystrengthened glass article using a masking technique. In particular, theprocess 1000 can be used to create adjacent zones having opposingasymmetrical chemical strengthening. In accordance with the embodimentsdescribed herein, the adjacent zones may help to control crackpropagation while also helping to maintain a flat part with a reducedtendency to warp or bend due to the asymmetric internal stresses.

In operation 1002, a first mask is positioned along a first surface of aglass article. If the glass article is a cover glass, the first surfacemay define at least a portion of an external surface of the electronicdevice. The first mask may form a coating over a portion of the firstsurface, such as a front surface of the cover glass. The first mask maybe a mask similar to as shown below with respect to FIG. 11A. Othermasking examples are described below with respect to FIGS. 11B, 14A-14C,15A, 16A, 16B, and 18A-18F. In embodiments, a first portion of the firstsurface remains uncoated by the first mask and a second portion of thefirst surface is coated by the first mask.

In operation 1004, a first compressive stress region is formed along thefirst surface by exchanging ions into the cover sheet. The firstcompressive stress region has a first thickness (or depth of layer). Forexample, the first compressive stress region may be formed along thefirst portion of the first surface. Operation 1004 may be performed inaccordance with any of the ion-exchange processes or techniquesdescribed herein. For example, the ion-exchange process described withrespect to FIGS. 2 and 3 may be used to perform operation 1004. However,other known ion-exchange techniques may also be used. The ion exchangemay be a single ion exchange or it may involve a series of ionexchanges. In embodiments, the ion exchange of operation 1004 maycomprise exchange of ions in the glass (e.g., lithium ions) for sodiumions in a first bath. In embodiments, the first bath may comprise asodium salt at a concentration greater than 50% mol. In additionalembodiments, the first bath may further comprise additional alkali metalions in a lesser amount.

In operation 1006, the first mask is removed. The first mask may beremoved using a solvent and/or mechanical technique. In operation 1008,a second compressive stress region adjacent to the first compressivestress region may be formed by exchanging ions into the cover sheet sothat the second compressive stress region has a second thickness that isless than the first thickness. For example, the second compressivestress region may be formed along the second portion of the firstsurface. Furthermore, the first compressive stress region formed inoperation 1004 may be modified by additional ion exchange duringoperation 1008. Similar to operation 1004, any one of the previouslydescribed ion-exchange techniques may be used to perform operation 1008.The time, temperature, ion concentration, or other characteristic orparameter of the ion-exchange process may be modified in order toproduce the desired second thickness. In embodiments, the ion exchangeof operation 1008 may comprise exchange of sodium ions in the glass forpotassium ions in a second bath. In further embodiments, lithium ionspresent in the glass may also be exchanged for potassium ions. Inembodiments, the second bath may comprise a potassium salt at aconcentration greater than 50% mol. In additional embodiments, thesecond bath may further comprise additional alkali metal ions in alesser amount.

In operation 1010, a second mask is positioned along a second surfacethat is opposite to the first surface. The second mask may form acoating over a portion of the second surface, such as a rear surface ofthe cover glass. The second mask may be similar or may differ in shapeand/or size from the first mask. In embodiments, the second mask may beoffset with respect to the first mask. As examples, the second mask mayonly partially overlap the first mask or the second mask may not overlapthe first mask. The second mask may be a mask similar to as shown belowwith respect to FIG. 11B. Other masking examples are described belowwith respect to FIGS. 11A, 14A-14C, 15A, 16A, 16B, and 18A-18F. Inembodiments, a first portion of the second surface remains uncoated bythe second mask and a second portion of the second surface is coated bythe second mask. In additional embodiments, the first portion of thesecond surface is offset with respect to the first portion of the firstsurface and the second portion of the second surface is offset withrespect to the second portion of the first surface. In operation 1012, athird compressive stress region is formed that extends from the secondsurface toward the second compressive stress region and has a thirdthickness. For example, the third compressive stress region may beformed along the first portion of the second surface. Operation 1012 maybe similar to the ion-exchange operation used to perform operation 1004.In embodiments, the ion exchange of operation 1012 may comprise exchangeof ions in the glass (e.g., lithium ions) for sodium ions in a bath.

In operation 1014, the second mask is removed. Operation 1014 may besimilar to operation 1006. In operation 1016, a fourth compressivestress region is formed that extends from the second surface toward thefirst compressive stress region and has a fourth thickness that is lessthan the third thickness. For example, the fourth compressive stressregion may be formed along the second portion of the second surface.Furthermore, the third compressive stress region formed in operation1012 may be modified by additional ion exchange during operation 1014.Operation 1016 may be performed in a similar fashion to as describedabove with respect to operation 1008. In embodiments, the ion exchangeof operation 1016 may comprise exchange of sodium ions in the glass forpotassium ions in a bath. In further embodiments, lithium ions presentin the glass may also be exchanged for potassium ions.

The above operations may be repeated multiple times in order to achievethe desired stress distribution throughout the glass article. It is notnecessary that all of the operations be performed in the sequencedescribed above and, in some cases, various operations may be combinedor performed simultaneously or during an overlapping time period. Forexample, the first and second masks could be applied and the first andthird compressive stress regions could be performed using the same bathor ion-exchange process. Similarly, the masks could be removed and thesecond and fourth compressive stress regions could be performed usingthe same bath or ion-exchange process. If operations are not to beperformed simultaneously, additional masks may be employed. For example,the second surface may be masked during operations 1004 and 1008 and thefirst surface may be masked during operations 1012 and 1016.

FIGS. 11A-11B depict example masking techniques and a resulting glassarticle. In one embodiment, as shown in FIG. 11A, a coating or filmcomposed on silicon nitride (e.g., SiN, SiN_(x), or Si₃N₄), or otherlike material, is used instead of a physical mask. In FIG. 11A, acoating 1100 is applied to the central portion of a cover glass 1102,while the edges and corners 1104 are left uncoated. Such a coating wouldlimit or eliminate ion diffusion at the center region or portion of thecover glass, while allowing chemical strengthening at the non-coatedregions (edges and corners). In an alternative embodiment, a coating ormask can be applied to the central portion and edges (a first surface),while leaving each of the four corners exposed. Chemical strengtheningat the non-coated regions (four corners) would occur. A second maskcould then cover the four corners (a second surface), followed bychemical strengthening.

The coating or mask is first applied to the cover glass prior to theenhancement treatment to block substantially all ion diffusion throughthe coated portion of the cover glass. Coatings or masks can have athickness of from about 5-500 nm, although other thicknesses may be usedwhere appropriate. In this illustration, the coated surface of the glassarticle, upon completion of the chemical strengthening process, wouldnot include a compressive stress region, whereas the remainder of thecover glass would exhibit a compressive stress region. Upon completionof the chemical strengthening process, the coating or mask could beremoved via polishing from the cover glass, providing a surface havingasymmetric strengthening, or could be left on the surface of the glass,as part of the finished glass article. In this aspect, the coating ormask would be tailored to an appropriate thickness and composition inorder to remain part of the cover glass.

In other embodiments, the silicon nitride coating can be oxidized afterthe chemical strengthening process is complete to provide a moreion-permeable barrier. The same cover glass may now be re-immersed andprocessed through chemical strengthening, such that some ion diffusionoccurs through the silicon dioxide barrier, and thereby some compressivestress region is formed at the locale (while the remainder of the coverglass has been treated twice).

As just noted, a coating composed of alternative materials, silicondioxide for example, can also be used to limit, rather than eliminate,ion diffusion to the surface of the glass. For example, a coatingcomposed of silicon dioxide would only limit ion diffusion to the coverglass surface, allowing some level of compressive stress regionformation in the coated region, but not the complete strengtheningcontemplated by the ion-exchange baths. As above, the coating would beeither removed upon completion of the chemical strengthening process, orleft in place as part of the finalized article. In either case, thecover glass would have a surface with asymmetric strengthening and apattern of asymmetric strengthening. These patterns can be used, alongwith glass density and heating (above), to form both strengthened andflat glass surfaces. Using these tools for asymmetric strengthening,patterns are applied to glass articles to form regions of increasedcompressive stress to protect the article for its intended use and tobalance out or oppose those forces to reduce or limit warpage on thecover glass. In addition, regions of tensile stress are positionedbetween these compressive stress regions that extend from the front andrear surfaces of the cover glass. The tensile stress regions are pushedbetween the front surface and rear surface compressive stress regions,and, as seen in FIG. 7C, are adjacent the lower compressive stressregion and pushed away from the higher and deeper compressive stressregion.

FIG. 11B shows combinations of coating types (1100, 1102, 1104) andthicknesses that can be used in designing an asymmetrically strengthenedglass surface. In FIG. 11B, a series of coatings or masks (1100, 1102,1104) are applied to both the front and rear surface (1106 and 1108,respectively) of a cover glass 1110. Each combination of coating ormasking material is meant to control ion diffusion to the target glasssurface, and thereby modify the chemical strengthening of that surface1112.

The cover glass can exchange and incorporate ions to a particular depthand concentration based on the ion diffusion through masks 1100, 1102and 1104. As described previously, the change in ion concentration alongthe surface, and to particular depths, modifies the glass internalstress relationship. In the stress pattern resulting from the maskconfiguration shown in FIG. 11B the edges 1116 of the front surface1106, having no mask, receive the most robust ion concentration alongthe surface, and to the greatest depth. The remainder of the frontsurface 1106 shows some reduced ion incorporation, but to a lower extentthan at the edges 1116. The rear surface 1108, being internal, forexample, has multiple regions defining three areas of ion incorporation1116, 1118, 1120, based on the layered coatings. The center region 1120of the rear surface has little or no ion incorporation due to masks1100, 1102, and 1104. The combined masks eliminate almost all iondiffusion into the center region. The other regions show some iondiffusion that result from either the single coating or the combinationcoating. Thus, a stress relationship where multiple masks (ion barriers)have been applied to prepare an asymmetrically strengthened cover glassis achieved. The combination of masks is applied to prepare theappropriate strengthening requirements, as well as oppose one anotherand form a cover glass having little or no warpage.

It is further envisioned that multiple layers of mask can also be usedto control the ion diffusion process into the target cover glasssurface. For example, a thin mask that limits sodium and potassium iondiffusion from a chemical strengthening process by 25% could be layeredacross a first thicker mask that limits sodium and potassium iondiffusion by 50%. The glass surface region would potentially have aregion limited of ion diffusion by 0% (no mask), 25% (first mask), 50%(second mask), and 75% (layered mask); other embodiments may havedifferent percentages for each mask. As above, the finished cover glasssurface could include each of the masking layers, or could be treated toremove the masks, leaving only the underlying asymmetricallystrengthening surfaces. It is also envisioned that the ion-diffusionbarrier coatings can be combined with the ion-barrier masks to furtherallow for calibrated glass article surface strengths—for example,physically mask a rear surface of the cover glass and coat patterns orlocales with a 25% ion diffusion barrier on the front surface of thecover glass.

The use of any of the above described asymmetric chemical strengtheningtools can be used to prepare a cover glass for its intended use, whilemaintaining flat surfaces (e.g., having an absence of warpage and beingno more than 100-120 μm out of plane). Parameters would first identifyregions of the cover glass in need of increased chemical strengtheningand then regions that can be used to oppose or limit the warpage formedby the required chemical strengthening zones. In some embodiments acover glass can have a front surface that includes a first compressivestress region that extends into the cover glass from the front surfaceto a first depth. The front surface also includes a second compressivestress region that extends into the cover glass from the front surfaceto a second depth. The second depth is less than the first depth. Theopposing rear surface has a third compressive stress region that extendsinto the cover glass from the rear surface toward the first compressivestress region and has a third depth. The opposing rear surface also hasa fourth compressive stress region that extends into the cover glassfrom the rear surface toward the second compressive stress region to afourth depth. The fourth depth is greater than the third depth. A firsttensile stress region is positioned between the first compressive stressregion and the third compressive stress region, and a second tensilestress region is positioned between the second compressive stress regionand the fourth compressive stress region.

As noted previously, asymmetric chemical strengthening results in anincrease in the concentration of exchanged ions within a treated zone ofa glass article. As a zone of glass is strengthened, the incorporationof ions can lead to formation of compressive forces within the glassthat can result in bending and loss of a substantially flat surface.

As shown in FIG. 12, chemically strengthening a top (front) surface 1200of a cover glass 1202 is highly useful for impact resistance and impactdamage protection. However, as schematically shown in thecross-sectional view of FIG. 12, an increased volume of ions 1203 in thetop surface edge tends to cause expansion of the front surface. Sincethe volume of ions 1203 is greater than that at the corresponding regionof rear surface 1210, downward deformation or curvature (schematicallyshown by arrow 1206) may occur at region 1204 and the result could be awarped cover glass. However, other regions 1208, for example, of thecover glass can be asymmetrically strengthened to oppose the deformation1206 with an opposite deformation (schematically shown by arrow 1207)from the rear edge 1210 of the cover glass. Region 1208 has received asimilar increase in volume of ions 1209 to deform the zone of glass inthe opposite direction, as shown by arrow 1207. The combined forcesallow the front and rear edges to remain substantially flat due to therelative balance of forces acting on the glass. Avoidance of warpage byincluding various patterned asymmetrically strengthened regions can beutilized such that a compressive force region in one direction can bepartially or fully compensated for by a compressive force region in adifferent zone of the glass. The pattern or overall combination ofchemically strengthened regions results in the cover glass havingsubstantially flat surfaces.

FIG. 13 shows an example of additional cover glass embodiments whichinclude a localized curve or bend. The localized curve or bend creates anon-planar or contoured cover glass profile or shape. The cover glassmay be formed or machined to create the localized curve or bend and thenon-planar profile or shape. In the following description, cover glass1300 of FIG. 13 is described as having a bend 1302. However, cover glass1300 may alternately be described as having a curve 1302.

As in the previous description, patterned asymmetric chemicalstrengthening can be utilized to ensure both safety to the cover glass1300 and minimize potential damage to the cover glass at the bend 1302.As shown in FIG. 13, an illustrative cross-sectional view of a coverglass 1300 shows that inclusion of patterned asymmetric chemicalstrengthening along the surfaces (front 1304 and rear 1306) of the coverglass 1300 can be maintained, except where the cover glass 1300 bends1302 and shows a thinning (shown by arrows 1308) of the cover glass1300. As shown in FIG. 13, the cover glass 1300 has a pattern of highcompressive stress regions 1310 and lower compressive stress regions1312 extending into the cover glass 1300 from the front 1304 and rear1306 surfaces. The front 1304 and rear 1306 surfaces may define acentral zone of the cover glass.

However, the depth of the compressive stress regions 1316 is minimizedat the bend 1302. For example, the depth of the compressive stressregions 1316 may be less than those of compressive stress regions 1310and 1312 to maintain the level of tensile stress 1314 below a desiredlevel. Given the reduced thickness of the cover glass 1300 at bend 1302,opposition of deeper compressive stress regions at the bend may undulyincrease the central tension at the bend. As shown in FIG. 13, thecompressive stress region 1316 at the bend 1302 is more symmetricbetween the front and rear surfaces and a middle of the glass centerlinemaintained by the resultant tensile stress region. In embodiments, thethickness of the cover glass (e.g., at the bend) is maintained at orabove a specified thickness so as not to unduly decrease the thicknessof the tensile stress region and unduly increase the central tension inthe glass.

In additional embodiments, an asymmetric chemical strengthening pattern,such as any of the asymmetric chemical strengthening patterns describedherein, may be applied to a central zone of a glass article including alocalized bend. The localized bend may be located between a central zoneand a peripheral zone of the glass article. The localized bend and theperipheral zone may be strengthened differently than the central zone asshown as FIG. 13.

As previously discussed in FIG. 7C, a stress profile at any one zone mayinclude an asymmetric stress profile for a glass article where the top(front) surface shows a more significant surface compressive stress CSand compressive stress layer depth (DoL) than the rear surface 1306.Note that the front surface 1304 would, in this case, be more durableand impact resistant than the rear surface 1306. The inclusion ofadditional compressive stress on the surface may be compensated for by amuch shallower depth of compression on the bottom (rear) surface. In theabsence of the compensation (e.g., if the stress profile were symmetricrather than asymmetric), the tensile forces may lead to an undesirablecentral tension value (tensile forces may extend undesirably far to theleft in the stress profile, potentially to a central tension value whichcould cause failure of the cover glass). In addition, the greaterexpansion of the top surface may cause both top and bottom surfaces tobegin to bend or curve. The result can be localized warpage of the topand bottom surfaces. However, as described herein, the stress patterncan compensate for the localized warpage of the top and bottom surfaces.

Compressive stress layers typically show a maximal level of stress at ornear the surface of the glass article, which decreases as the ionsmigrate deeper into the glass. At the surface of a surface compressivestress layer, the compressive stress force can be up to 750 MPa anddecrease to zero over the distance of 100 to 150 μm into the glass.Other compressive stress versus depth of compression ratios can beutilized dependent on the need and type of asymmetric chemicalstrengthening used, for example, for the densified glass shown in FIG. 6(see region 602). Impact at a zone in the glass article having 750 MPacompressive stress at the surface would likely not allow for crackformation or propagation, whereas impact at a zone on the surface having150 MPa, for example, very possibly could. It is the patterning of theseforces that allow for a reduction or limitation of warpage and forpreparation of a pathway for crack propagation.

FIGS. 14A, 14B, and 14C depict examples of masks applied to the frontand rear surface of a cover glass. For example, the mask configurationdepicted in FIG. 14A can be used to produce adjacent zones with opposingasymmetric stress profiles such as depicted in FIGS. 7C and 7D. Asdepicted in FIG. 14A, the masks 1432 cover the portions 1414 a of thefront surface 1410 of the cover glass 1400 a, leaving the portion 1412 auncovered. The mask 1442 covers the portion 1424 a of the rear surface1420 of cover glass and leaves the portion 1422 a uncovered.

An example chemical strengthening process includes two ion-exchangeoperations, with the mask being removed between the two ion-exchangeoperations. During a first ion-exchange operation, ions from the firstion-exchange bath will diffuse deepest into the glass from the portions1412 a and 1422 a, which are uncovered by the masks 1432 and 1442. As aresult, ion-exchanged regions resulting from the first ion-exchangeoperation extend deeper from portions 1412 a and 1422 a than fromportions 1414 a and 1424 a. In embodiments, the ion-exchanged regions donot substantially extend under portions 1414 a and 1424 a after thefirst ion-exchange operation. In further embodiments, the ion-exchangedregions extend under portions 1414 a and 1424 a (e.g., under the edgesof the mask), but to a lesser extent than under portions 1412 a and 1422a.

After removal of the masks 1432 and 1442, a second ion-exchangeoperation introduces ions from the second ion-exchange bath alongportions 1412 a, 1414 a, 1422 a, and 1424 a. The ion-exchanged regionsresulting from the first ion-exchange operation are modified during thesecond ion-exchange operation. The first and second ion-exchangeoperations may form an ion-exchanged layer along each of the frontsurface 1410 and the rear surface 1420 of the cover glass. Inembodiments, the depth of the ion-exchanged layer will vary along eachof the front and the rear surface and have a greater depth under theregions 1412 a and 1422 a.

The variation in the depth of the ion-exchanged layer along the frontand/or rear surface leads to a variation in the depth of the compressivestress layer along the front and/or rear surface. For example, regionsof the compressive stress layer extending from portions of the front orrear surface which were not covered by the mask may have a greater depthof compressive stress layer, as schematically illustrated in FIG. 15A.The zone of the cover glass defined between surfaces 1412 a and 1424 ais asymmetrically strengthened, as is the zone defined between surfaces1414 a and 1422 a. However, the asymmetry of these two zones isopposite, with the deeper compressive stress regions formed at theportion 1412 a of the front surface 1410 and at the portion 1422 a ofthe rear surface 1420.

The mask configuration depicted in FIG. 14B can be used to produce azone with an asymmetric stress profile adjacent to a zone with asymmetric stress profile. As depicted in FIG. 14B, the masks 1432 coverthe portions 1414 b of the front surface 1410 of the cover glass 1400 b,leaving the portion 1412 b uncovered. The mask 1444 covers the portion1424 b of the rear surface 1420 of the cover glass. The zone definedbetween portions 1412 b and 1424 b may be strengthened asymmetricallywhile the zone defined between portions 1414 b and 1424 b may bestrengthened symmetrically.

The mask configuration depicted in FIG. 14C can be used to produce aseries of adjacent zones with opposing asymmetric stress profiles. Asdepicted in FIG. 14C, the masks 1436 cover the portions 1414 c of thefront surface 1410 of the cover glass 1400 c, leaving the portions 1412c uncovered. The masks 1446 cover the portions 1424 c of the rearsurface 1420 of the cover glass and leave the portions 1422 c uncovered.The zones of the cover glass defined between portions 1412 c and 1424 care asymmetrically strengthened, as are the zones defined betweenportions 1414 c and 1422 c. However, the asymmetry of these two zones isopposite, as previously described for FIG. 14A. As shown in FIG. 14C,both the size of the masks and the spacing of the masks along a givensurface of the cover glass may be substantially uniform. In additionalembodiments, the size of the masks and/or the spacing of the masks mayvary in order to provide the desired compressive pattern.

As described for FIGS. 14A-14C, deeper ion-exchanged layers may beformed along regions of the cover glass surface which are not covered bya mask. In alternate embodiments, a first set of masks may be used toform the deeper ion-exchanged regions and a second set of masks may beused to form the shallower ion-exchanged regions. The first set of masksmay cover first portions of the cover glass surface and the second setof masks may cover second portions of the surface of the cover glasssurface. In some embodiments, the first portions of the cover glasssurface may partially overlap the second portions of the cover glasssurface. In further embodiments, the first portions of the cover glasssurface may not overlap the second portions of the cover glass surface.

FIG. 15A shows a representative cross-sectional view of a glass article1500 having a series of asymmetrically strengthened zones 1502 along thefront 1504 and rear 1506 surfaces, showing how a pattern ofstrengthening on the front and rear surfaces can be used to maintainrelatively flat surfaces. In this example, a pattern of high and lowcompressive stress at the front surface 1504 opposes the oppositepattern of low and high compressive stress at the rear surface 1506. Inthis example, the pattern extends across the entirety of the glassarticle 1500. For simplicity, compressive stress regions are not shownin FIG. 15A as extending along side surfaces 1508. However, inadditional embodiments compressive stress regions may formed along sidesurfaces 1508.

FIG. 15A also schematically depicts the positions of masks 1532 ofmaterial along the front surface 1504 during an ion-exchange operation.The masks 1532 can be used during chemical strengthening to limit iondiffusion along the front surface 1504, where a shallow or low surfacecompression 1510 is required. In between areas of masking, deeper andmore robust ion diffusion can occur and allow for a deeper surfacecompression region 1512. As depicted in FIG. 15A, the transition betweena compressive stress region having a greater depth of layer and anadjacent compressive stress region having a lesser depth of layer may bea gradual transition rather than a step transition. Additionaltechniques as described above can be used to prepare different zones ofcompressive stress and, therefore, different patterns of compressivestress, for example, densified glass.

As depicted in FIG. 15A, an opposite pattern is achieved on the rearsurface of the same glass article, such that a high compressive stressfront region is opposed by a low surface compressive rear region, andvice versa. The alternation of stress patterns across the front and rearsurfaces work to oppose each other, so as to yield a glass articlehaving substantially flat front and rear surfaces. Any number and shapedregions can be utilized to form a pattern, as long as the combinationsof forces act to result in a substantially flat surface.

FIG. 15A depicts an example pattern of adjacent asymmetric stressregions that form complementary pairs. In particular, as shown in FIG.15A, the glass article 1500 (e.g., a cover glass) includes a firstcompressive stress region 1551 that extends into the glass article 1500from the front surface 1504 a first depth. A second compressive stressregion 1552 extends into the glass article 1500 from the front surface1504 a second depth that is less than the first depth. A thirdcompressive stress region 1553 extends into the glass article 1500 fromthe rear surface 1506 toward the first compressive stress region 1551 athird depth. A fourth compressive stress region 1554 extends into theglass article 1500 from the rear surface 1506 toward the secondcompressive stress region 1552 a fourth depth that is greater than thethird depth. In some cases, the first depth is approximately equal tothe fourth depth, and the second depth is approximately equal to thethird depth. In some cases, the first depth is at least twice the seconddepth and the fourth depth is at least twice the third depth. Inembodiments, the first depth may be a maximum depth of the firstcompressive stress region and the fourth depth may be a maximum depth ofthe fourth compressive stress region. In addition, the second depth maybe a minimum depth of the second compressive stress region and the thirddepth may be a minimum depth of the third compressive stress region. Insome instances, the first compressive stress region 1551 and the secondcompressive stress region 1552 may also be referred to as a (first orsecond) front localized compressive stress region. Similarly, the thirdcompressive stress region 1553 and fourth compressive stress region 1554may also be referred to as a (first or second) rear localizedcompressive stress region. In some cases, the first (localized)compressive stress region 1551 is at least partially surrounded by thesecond compressive stress region 1552.

In embodiments, the compressive stress regions are formed using twoion-exchange operations with an intermediate operation of removing themask(s) from the glass article. For example, a first ion-exchangeoperation comprises exchange of lithium ions in the glass for sodiumions in a first ion-exchange bath and a second ion-exchange operationcomprises exchange of sodium ions (and optionally lithium ions) in theglass for potassium ions in a second ion-exchange bath. The potassiumions introduced by ion exchange may extend into the glass article tosubstantially the same depth along the front and the rear surfaces ofthe glass article. However, the sodium ions introduced by ion exchangemay extend to greater depths from portions of the front and the rearsurface which were not covered by masks (e.g., 1532). As a result, eachof the respective compressive stress regions 1551, 1552, 1553, and 1554include potassium ions. The greater depth of compressive stress regions1551 and 1554 may be due to the greater depth of sodium ion diffusioninto the glass article during the first and second ion-exchangeoperations.

In some instances, the process to form the compressive stress regionsmay include an ion-exchange operation comprising ion exchange of ions inthe glass for potassium ions in an ion-exchange bath before removal ofthe mask(s) from the glass article. In embodiments, the process furtherincludes an additional ion-exchange operation comprising exchange ofions in the glass for potassium ions in an ion-exchange bath afterremoval of the mask(s) from the glass article. In embodiments, each ofthe respective compressive stress regions 1551, 1552, 1553, 1554 includepotassium ions that extend into the glass article 1500 to a depth thatcorresponds to the respective depths depicted in FIGS. 15A-15C. In somecases, the first (localized) compressive stress region 1551 is at leastpartially surrounded by the second compressive stress region 1552. Thefirst (localized) compressive stress region 1551 includes potassium ionsthat extend into the enclosure at a first depth and the secondcompressive stress region 1552 includes potassium ions that extend intothe enclosure at a second depth that is less than the first depth. Insome cases, the first depth is at least twice the second depth.

FIG. 15A also schematically depicts a first tensile stress region 1561positioned between the first compressive stress region 1551 and thethird compressive stress region 1553 and a second tensile stress region1562 positioned between the second compressive stress region 1552 andthe fourth compressive stress region 1554. As schematically shown in theFIGS. 15A and 15C, a first centerline 1566 of the first tensile stressregion 1561 is offset with respect to a second centerline 1568 of thesecond tensile stress region 1562. In addition, the depth of the maximumcentral tension of the first tensile stress region may be offset withrespect to the depth of the maximum central tension of the secondtensile stress region, as schematically shown in FIGS. 7C and 7D. In analternative embodiment, a first tensile stress region is formed betweenthe first compressive stress region and the fourth compressive stressregion. A second tensile stress region is formed between the secondcompressive stress region and the third compressive stress region, suchthat the first tensile stress region is offset with respect to acenterline of the cover sheet in a first direction and the secondtensile stress region is offset with respect to the centerline in asecond direction that is opposite to the first direction.

As shown in FIG. 15B the corresponding areas of tensile stress 1514 arein the central region of the glass (between the compressive stress ofthe asymmetrically strengthened zones 1502 along the front and rearsurfaces). Note that the tensile stress is located between opposingfront and rear surfaces, where the tensile stress is pushed away fromthe deeper and higher surface compression. A pattern of surfacecompression stress and tensile stress extends between a front 1504 andrear 1506 surface of the glass article. The combination of forces withinthe glass article can be used to both limit damage to the glass article,as well as maintain the glass article's flat surfaces. Note that thecenterline of tensile stress 1568 within the tensile stress area 1514changes with respect to the differing compressive stress regions.

In addition, by utilizing the surface compression stress and resultanttensile stress, impact or damage induced cracks can be controlled tofollow paths of lower compressive stress, higher tensile stress, orcombinations thereof. A glass article can be patterned using asymmetricchemical strengthening to control and direct impact at a corner or edgeaway from priority areas of the glass surfaces, and toward areas where acrack is of less visibility, for example.

FIG. 15C shows a cross-sectional view of glass that exhibits tensilestress 1516 between the asymmetrically strengthened zones 1502 along thetwo surfaces 1504, 1506. Areas of tensile stress 1516 are resident basedon the asymmetry of the compressive stress exerted by the front and rearsurfaces. Impacts (shown as arrow 1518) at a front surface of glassmigrate (line 1520) and propagate away from higher surface compressionand toward areas of more accessible tensile stress. Areas of relativelylow tensile stress can be utilized to capture impact propagation.Further, areas of tensile stress can be utilized to turn or deviate acrack toward the tensile stress and away from the higher surfacecompression. As such, positioning of the tensile stress provides avenuesfor directing or turning impact crack propagation toward zones in theglass of lower priority.

As shown in FIG. 15D, the dimensions and stress levels of thecompressive stress regions in a glass article may be adjusted to provideprotection to priority areas from impact crack propagation. For example,a camera 1526 may be positioned below a camera window 1524. A localizedcompressive stress region extending from the front surface 1504 in thezone defined by the camera window 1524 has a level of compressive stressthat is greater than that of adjacent localized compressive stressregions. In addition, a width of the localized compressive stress regionextending from the front surface 1504 in the zone defined by the camerawindow 1524 may be different from that of the adjacent localizedcompressive stress regions. As shown in FIG. 15D, the width of thelocalized compressive stress region extending from the front surface1504 in the zone defined by the camera window 1524 is greater than thatof the adjacent localized compressive stress regions. Similarly, thewidth of the localized compressive stress region extending from the rearsurface in the zone defined by the camera window 1524 is greater thanthat of the adjacent localized compressive stress regions. Inembodiments, the desired relative widths of the localized compressivestress regions may be achieved by adjusting the size and spacing of themasks applied to the front and rear surfaces of the glass article.

The pattern of stresses shown in FIG. 15D can be utilized to directimpact anticipated in that area of the glass article. For example,impact at a corner can be designed to direct a crack 1528 (due to, forexample, impact damage) into a region having opposing and offsetcompressive stress regions, which may arrest or redirect the crack 1528.Referring to FIG. 15D, impact (shown by arrow 1523) along a frontsurface 1504 of the cover glass sufficient to begin a crack is directedtoward and through regions of lower surface compression, and towardavailable internal areas of tensile stress 1522. In this example, impactpropagation toward the internal tensile stress would be developed toavoid a camera window 1524. Note that regions of densified glass can beincluded to add high volumes of compressive stress at a surface, buthaving much lower compressive depth and therefore changed tensilestress. The combinations of these forces are used to direct crackpropagation. The same principle may be applied to any transparent windowused for an optical sensor, a display, or other optical or visualcomponent.

FIGS. 16A and 16B show simplistic front (FIG. 16A) 1601 and rear (FIG.16B) 1603 surface views of alternating high and low surface compression.High-surface compression regions 1600 correspond to hatched areas andlow-surface compression regions 1602 correspond to non-hatched zones. Insome cases, the high-surface compression regions 1600 have a greaterthickness or depth as compared to the low-surface compression regions1602. Since any one cross-sectional area of glass thickness can onlyinclude a defined amount of volume, asymmetric chemical strengthening onone surface is typically paired with the other surface having a lesseramount of chemical strengthening. As discussed above, the positioning ofopposing stress regions is used to balance out each surface and provideflat surfaces. Any type of stress pattern can be used as long as thestress over the front and/or rear surfaces of the glass article opposeeach other and result in substantially flat or flat surfaces. Patternssuch as checkerboard, cross-hatch, crisscross, and the like can be inputinto the front, rear, or front and rear to balance the overall stressforces to a sufficient level that the surfaces are flat.

FIG. 17A shows a schematic of a cover glass having a region 1702 for acamera window and a combined stress profile or pattern used to directimpact damage. In FIG. 17A, avoidance of crack propagation through orwithin the camera window 1703 view line of the cover glass is importantfor overall utility. In embodiments, a region 1704 of relatively highcompressive stress is provided on the front surface of the glass articlearound the area where the camera window 1703 is positioned. Inembodiments, the region 1704 may have an annular or ring-shaped form. Asshown in FIG. 17A, an impact-related crack 1706 would not propagatethrough the high-surface compression region 1704, but rather through anadjoining low-surface compression region 1708. For example, a crack 1706that forms at the edge 1710 of a cover glass can be directed both awayfrom prioritized viewing areas 1712 (centrally located) and away fromthe camera window 1703. In this way, an impact crack can be anticipatedand minimized by the use of patterned asymmetric chemical strengthening.

FIG. 17B shows a pattern for directing impact damage from a corner 1750of a cover glass by using a series of compressive stress regions 1752,1754. In particular, a series of low-surface compression andcorresponding central tension zones formed as a result of compressivestress regions 1752, 1754 are configured to redirect a propagating crackfrom the corner 1750 and change the direction of the crack fromextending to the center 1756 of the cover glass to extending back towardthe edges 1758 of the cover glass. A series of such regions can bepositioned such that excessive impact force that continues past thefirst of these regions can be caught and redirected by the second ofthese regions. The pattern can be continued to provide one or more, twoor more, three or more, or four or more such regions that radiate fromeach corner of the glass article.

Alternative patterns of asymmetric chemical stress are shown in FIG.18A-18F. In FIGS. 18A-18F, asymmetric chemical strengthening patternsare shown for enhancing the strength of the corner and/or edge zones ofa cover glass, while allowing opposing stress to offset for warpage onthe opposite surfaces.

FIG. 18A and FIG. 18B show a front surface 1800 (FIG. 18A) and rearsurface 1802 (FIG. 18B) or an illustrative cover glass, the cover glassdefining a rectangular outer perimeter region. The hatched zones 1804are located at least partially within one of the four corner regions ofthe cover glass, and show a first localized compressive stress regionthat extends into the cover glass from the surface to a first depth. Asecond localized compressive stress region 1806 extends into the coverglass from the front surface a second depth that is less than the firstdepth. On the rear surface 1802, a third localized compressive stressregion 1808 extends into the cover glass from the rear surface towardthe first localized compressive stress region 1804 to a third depth. Afourth localized compressive stress region 1810 extends into the coverglass from the rear surface toward the second localized compressivestress region 1806 to a fourth depth, the fourth depth greater than thethird depth. Note that in this embodiment, the second compressive stressregion 1806 surrounds the first localized compressive stress region1804. Also note that although not shown, embodiments include havinglocalized compressive stress regions only partially surround otherlocalized compressive stress regions.

As shown in FIGS. 18A and 18B, the compressive stress is patterned tooppose each other, from the front and rear surfaces, to avoid warpage.

A similar pattern is shown in FIG. 18C 1850 (front) and FIG. 18D 1852(rear). A cover glass defines four corner regions 1853. Here, asemi-circular first compressive stress region 1854 located at a corner1853 of the front surface, extends into the cover glass from the frontsurface 1850 to a first depth, and a second compressive stress region1856 extends into the cover glass from the front surface to a seconddepth. In this example, the first depth is greater than the second depthso as to provide impact damage resistance. A third compressive stressregion 1858 extends into the cover glass from the rear surface 1852toward the first compressive stress region 1854 to a third depth. Afourth compressive stress region 1860 extends into the cover glass fromthe rear surface toward the second compressive stress region 1856 to afourth depth. In this example, the fourth depth is greater than thethird depth. Note also that the fourth compressive stress region 1860does not extend over the remainder of the rear surface, but rather onlyto the extent to limit or avoid warpage and control impact propagation.Also note that the first compressive stress region 1854 and the thirdcompressive stress region 1858 are located at least partially within oneof the four corner regions 1853 of the cover glass.

Finally, in FIG. 18E 1870 (front) and FIG. 18F 1872 (rear) surfaces fora cover glass are illustrated. In this design, a first compressivestress region 1874 extends around the rectangular perimeter edge (orouter perimeter region) 1873 of the front surface 1870. The firstcompressive stress region 1874 extends into the cover glass from thefront surface 1870 to a first depth. A second compressive stress region1876, adjacent to the first compressive stress region 1874, extends intothe cover glass from the front surface to a second depth. The firstdepth is greater than the second depth to improve the impact resistanceof the front surface along the edge 1873. As noted above, the rearsurface 1872 opposes the front surface 1870. A third compressive stressregion 1878 extends into the cover glass from the rear surface 1872toward the first compressive stress region 1874 to a third depth. Afourth compressive stress region 1880 extends into the cover glass fromthe rear surface 1872 toward the second compressive stress region 1876to a fourth depth. The fourth depth is greater than the third depth. Ineach design of FIGS. 18A-18F, the cover glass has added strength inregions where impact damage tends to accumulate 1804, 1854, 1874 andwarpage is avoided by opposing stress on the opposite face of the glass1808, 1858, 1878.

In an alternative embodiment, a cover glass for use in an electronicdevice, for example, could have a front localized compressive stressregion which is at least partially surrounded by a second localizedcompressive stress region. The first localized compressive stress regionincludes potassium ions that extend into the enclosure at a first depth.The second localized compressive stress region includes potassium ionsthat extend into the enclosure at a second depth that is less than thefirst depth, and the first depth is at least twice the second depth.

FIG. 19 is a block diagram of example components of an exampleelectronic device. The schematic representation depicted in FIG. 19 maycorrespond to components of the devices depicted in FIG. 1A-18F asdescribed above. However, FIG. 19 may also more generally representother types of electronic devices with a strengthened glass component asdescribed herein.

In embodiments, an electronic device 1900 may include sensors 1920 toprovide information regarding configuration and/or orientation of theelectronic device in order to control the output of the display. Forexample, a portion of the display 1914 may be turned off, disabled, orput in a low energy state when all or part of the viewable area of thedisplay 1914 is blocked or substantially obscured. As another example,the display 1914 may be adapted to rotate the display of graphicaloutput based on changes in orientation of the device 1900 (e.g., 90degrees or 180 degrees) in response to the device 1900 being rotated. Asanother example, the display 1914 may be adapted to rotate the displayof graphical output in response to the device 1900 being folded orpartially folded, which may result in a change in the aspect ratio or apreferred viewing angle of the viewable area of the display 1914.

The electronic device 1900 also includes a processor 1904 operablyconnected with a computer-readable memory 1902. The processor 1904 maybe operatively connected to the memory 1902 component via an electronicbus or bridge. The processor 1904 may be implemented as one or morecomputer processors or microcontrollers configured to perform operationsin response to computer-readable instructions. The processor 1904 mayinclude a central processing unit (CPU) of the device 1900. Additionallyand/or alternatively, the processor 1904 may include other electroniccircuitry within the device 1900 including application specificintegrated chips (ASIC) and other microcontroller devices. The processor1904 may be configured to perform functionality described in theexamples above. In addition, the processor or other electronic circuitrywithin the device may be provided on or coupled to a flexible circuitboard in order to accommodate folding or bending of the electronicdevice.

The memory 1902 may include a variety of types of non-transitorycomputer-readable storage media, including, for example, read accessmemory (RAM), read-only memory (ROM), erasable programmable memory(e.g., EPROM and EEPROM), or flash memory. The memory 1902 is configuredto store computer-readable instructions, sensor values, and otherpersistent software elements.

The electronic device 1900 may include control circuitry 1906. Thecontrol circuitry 1906 may be implemented in a single control unit andnot necessarily as distinct electrical circuit elements. As used herein,“control unit” will be used synonymously with “control circuitry.” Thecontrol circuitry 1906 may receive signals from the processor 1904 orfrom other elements of the electronic device 1900.

As shown in FIG. 19, the electronic device 1900 includes a battery 1908that is configured to provide electrical power to the components of theelectronic device 1900. The battery 1908 may include one or more powerstorage cells that are linked together to provide an internal supply ofelectrical power. The battery 1908 may be operatively coupled to powermanagement circuitry that is configured to provide appropriate voltageand power levels for individual components or groups of componentswithin the electronic device 1900. The battery 1908, via powermanagement circuitry, may be configured to receive power from anexternal source, such as an alternating current power outlet. Thebattery 1908 may store received power so that the electronic device 1900may operate without connection to an external power source for anextended period of time, which may range from several hours to severaldays. The battery 1908 may be flexible or attached to a flexiblesubstrate or carrier to accommodate bending or flexing of the electronicdevice.

In some embodiments, the electronic device 1900 includes one or moreinput devices 1910. The input device 1910 is a device that is configuredto receive input from a user or the environment. The input device 1910may include, for example, a push button, a touch-activated button,capacitive touch sensor, a touch screen (e.g., a touch-sensitive displayor a force-sensitive display), capacitive touch button, dial, crown, orthe like. In some embodiments, the input device 1910 may provide adedicated or primary function, including, for example, a power button,volume buttons, home buttons, scroll wheels, and camera buttons.

The device 1900 may also include one or more sensors 1920, such as aforce sensor, a capacitive sensor, an accelerometer, a barometer, agyroscope, a proximity sensor, a light sensor, or the like. The sensors1920 may be operably coupled to processing circuitry. In someembodiments, the sensors 1920 may detect deformation and/or changes inconfiguration of the electronic device and be operably coupled toprocessing circuitry which controls the display based on the sensorsignals. In some implementations, output from the sensors 1920 is usedto reconfigure the display output to correspond to an orientation orfolded/unfolded configuration or state of the device. Example sensors1920 for this purpose include accelerometers, gyroscopes, magnetometers,and other similar types of position/orientation sensing devices. Inaddition, the sensors 1920 may include a microphone, acoustic sensor,light sensor, optical facial recognition sensor, or other types ofsensing device.

In some embodiments, the electronic device 1900 includes one or moreoutput devices 1912 configured to provide output to a user. The outputdevice 1912 may include display 1914 that renders visual informationgenerated by the processor 1904. The output device 1912 may also includeone or more speakers to provide audio output. The output device 1912 mayalso include one or more haptic devices that are configured to produce ahaptic or tactile output along an exterior surface of the device 1900.

The display 1914 may include a liquid-crystal display (LCD),light-emitting diode, organic light-emitting diode (OLED) display, anactive layer organic light emitting diode (AMOLED) display, organicelectroluminescent (EL) display, electrophoretic ink display, or thelike. If the display 1914 is a liquid-crystal display or anelectrophoretic ink display, the display 1914 may also include abacklight component that can be controlled to provide variable levels ofdisplay brightness. If the display 1914 is an organic light-emittingdiode or organic electroluminescent type display, the brightness of thedisplay 1914 may be controlled by modifying the electrical signals thatare provided to display elements. In addition, information regardingconfiguration and/or orientation of the electronic device may be used tocontrol the output of the display as described with respect to inputdevices 1910. In some cases, the display is integrated with a touchand/or force sensor in order to detect touches and/or forces appliedalong an exterior surface of the device 1900.

The electronic device 1900 may also include a communication port 1916that is configured to transmit and/or receive signals or electricalcommunication from an external or separate device. The communicationport 1916 may be configured to couple to an external device via a cable,adaptor, or other type of electrical connector. In some embodiments, thecommunication port 1916 may be used to couple the electronic device to ahost computer.

The electronic device 1900 may also include at least one accessory 1918,such as a camera, a flash for the camera, or other such device. Thecamera may be connected to other parts of the electronic device 1900such as the control circuitry 1906.

The following discussion applies to the electronic devices describedherein to the extent that these devices may be used to obtain personallyidentifiable information data. It is well understood that the use ofpersonally identifiable information should follow privacy policies andpractices that are generally recognized as meeting or exceeding industryor governmental requirements for maintaining the privacy of users. Inparticular, personally identifiable information data should be managedand handled so as to minimize risks of unintentional or unauthorizedaccess or use, and the nature of authorized use should be clearlyindicated to users.

Other examples and implementations are within the scope and spirit ofthe disclosure and appended claims. For example, features implementingfunctions may also be physically located at various positions, includingbeing distributed such that portions of functions are implemented atdifferent physical locations. Also, as used herein, including in theclaims, “or” as used in a list of items prefaced by “at least one of”indicates a disjunctive list such that, for example, a list of “at leastone of A, B, or C” means A or B or C or AB or AC or BC or ABC (i.e., Aand B and C). Further, the term “exemplary” does not mean that thedescribed example is preferred or better than other examples.

The following discussion applies to the electronic devices describedherein to the extent that these devices may be used to obtain personallyidentifiable information data. It is well understood that the use ofpersonally identifiable information should follow privacy policies andpractices that are generally recognized as meeting or exceeding industryor governmental requirements for maintaining the privacy of users. Inparticular, personally identifiable information data should be managedand handled so as to minimize risks of unintentional or unauthorizedaccess or use, and the nature of authorized use should be clearlyindicated to users.

The foregoing description, for purposes of explanation, uses specificnomenclature to provide a thorough understanding of the describedembodiments. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the describedembodiments. Thus, the foregoing descriptions of the specificembodiments described herein are presented for purposes of illustrationand description. They are not targeted to be exhaustive or to limit theembodiments to the precise forms disclosed. It will be apparent to oneof ordinary skill in the art that many modifications and variations arepossible in view of the above teachings.

What is claimed is:
 1. A cover glass for an electronic device, the coverglass comprising: a front surface; a first compressive stress regionextending from the front surface to a first depth into the cover glass;a second compressive stress region extending from the front surface to asecond depth, less than the first depth, into the cover glass; a rearsurface opposite to the front surface; a third compressive stress regionextending from the rear surface toward the first compressive stressregion and to a third depth into the cover glass; and a fourthcompressive stress region extending from the rear surface toward thesecond compressive stress region and to a fourth depth, greater than thethird depth, into the cover glass.
 2. The cover glass of claim 1,further comprising: a first tensile stress region positioned between thefirst compressive stress region and the third compressive stress region;and a second tensile stress region positioned between the secondcompressive stress region and the fourth compressive stress region. 3.The cover glass of claim 2, wherein a first centerline of the firsttensile stress region is offset with respect to a second centerline ofthe second tensile stress region.
 4. The cover glass of claim 1,wherein: the second compressive stress region at least partiallysurrounds the first compressive stress region; and the fourthcompressive stress region at least partially surrounds the thirdcompressive stress region.
 5. The cover glass of claim 1, wherein: thefirst depth is approximately equal to the fourth depth; and the seconddepth is approximately equal to the third depth.
 6. The cover glass ofclaim 1, wherein: the cover glass defines four corner regions; and thefirst compressive stress region and the third compressive stress regionare located at least partially within one of the four corner regions ofthe cover glass.
 7. The cover glass of claim 1, wherein: the cover glassdefines a rectangular outer perimeter region; the first compressivestress region and the third compressive stress region are located atleast partially within the rectangular outer perimeter region; and thefirst compressive stress region at least partially surrounds the secondcompressive stress region.
 8. An electronic device comprising: adisplay; and an enclosure at least partially surrounding the display andcomprising: a first localized compressive stress region extending intothe enclosure from a front surface of the enclosure to a first depth; asecond localized compressive stress region adjacent to the firstlocalized compressive stress region and extending into the enclosurefrom the front surface to a second depth, less than the first depth; anda rear localized compressive stress region extending into the enclosurefrom a rear surface of the enclosure towards the second localizedcompressive stress region.
 9. The electronic device of claim 8, wherein:the rear localized compressive stress region is a third localizedcompressive stress region; the enclosure further comprises a fourthlocalized compressive stress region positioned adjacent to the thirdlocalized compressive stress region and extending towards the firstlocalized compressive stress region; a first tensile stress region ispositioned between the first localized compressive stress region and thefourth localized compressive stress region; and a second tensile stressregion is positioned between the second localized compressive stressregion and the third localized compressive stress region.
 10. Theelectronic device of claim 9, wherein the third localized compressivestress region extends to a third depth, greater than the second depth,into the enclosure.
 11. The electronic device of claim 9, wherein thefirst localized compressive stress region is at least partiallysurrounded by the second localized compressive stress region.
 12. Theelectronic device of claim 8, wherein: the enclosure defines a camerawindow; the electronic device further comprises a camera positionedbelow the camera window; the first localized compressive stress regionis positioned at least partially within the camera window; and thesecond localized compressive stress region surrounds the first localizedcompressive stress region.
 13. The electronic device of claim 8,wherein: the enclosure is a monolithic glass component; and themonolithic glass component defines at least the front surface and therear surface of the enclosure.
 14. The electronic device of claim 8,wherein: the enclosure includes a cover sheet having a front surface anda rear surface; the first localized compressive stress region and thesecond localized compressive stress region extend into the cover sheetfrom the front surface of the cover sheet; and the rear localizedcompressive stress region extends into the cover sheet from the rearsurface of the cover sheet.
 15. The electronic device of claim 14,wherein: the cover sheet has a length of at least 100 mm and a width ofat least 40 mm; and the front surface of the cover sheet has a flatnessof no more than 120 μm out of plane.
 16. A method of forming a coversheet for an electronic device, the method comprising: positioning afirst mask along a first surface defining at least a portion of anexternal surface of the electronic device; forming a first compressivestress region having a first thickness along the first surface byexchanging ions into the cover sheet; removing the first mask; forming asecond compressive stress region having a second thickness, less thanthe first thickness, adjacent to the first compressive stress region byexchanging ions into the cover sheet; positioning a second mask along asecond surface opposite the first surface; forming a third compressivestress region having a third thickness and extending from the secondsurface toward the second compressive stress region by exchanging ionsinto the cover sheet; removing the second mask; and forming a fourthcompressive stress region having a fourth thickness, less than the thirdthickness, and extending from the second surface toward the firstcompressive stress region by exchanging ions into the cover sheet. 17.The method of claim 16, wherein: the cover sheet comprises aluminasilicate glass; and forming the first compressive stress regioncomprises: immersing the cover sheet into a first bath comprising sodiumions; and subsequent to immersing the cover sheet in the first bath,immersing the cover sheet in a second bath comprising potassium ions.18. The method of claim 17, wherein: the first bath includes a sodiumconcentration of greater than 30% mol; and the second bath includes apotassium concentration of greater than 30% mol.
 19. The method of claim16, wherein: the cover sheet defines four corners; the first mask leaveseach of the four corners exposed along the first surface; and the secondmask covers each of the four corners along the second surface.
 20. Themethod of claim 16, wherein: a first tensile stress region is formedbetween the first compressive stress region and the fourth compressivestress region; a second tensile stress region is formed between thesecond compressive stress region and the third compressive stressregion; the first tensile stress region is offset with respect to acenterline of the cover sheet in a first direction; and the secondtensile stress region is offset with respect to the centerline in asecond direction opposite to the first direction.